Optical head device, inclination detection apparatus using the same, and optical information processing apparatus using the same

ABSTRACT

An optical head device includes a light source for emitting at least one of a coherent beam and a quasi-monochromatic beam; a collection optical system for collecting the beam to an information memory medium having a track which has at least one mark and at least one space; a light detector having a plurality of detection areas for receiving the beam reflected by the information memory medium and outputting a signal in accordance with a light amount of the beam received; and a tracking error signal generator for receiving the signals output from the light detector and generating a tracking error signal based on the signals. The tracking error signal generator reduces a difference between a first signal amplitude and a second signal amplitude. The first signal amplitude is an absolute value of a difference between first and second signal levels. The second signal amplitude is an absolute value of a difference between the first and third signal levels. The first signal level is a value of the tracking error signal obtained when the beam emitted by the light source is radiated to a center of the track. The second and third signal levels are, respectively, a maximum value and a minimum value of the tracking error signal obtained when the information memory medium is scanned by the beam emitted by the light source in a direction perpendicular to the track.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical head device for recordinginformation to, or reproducing or erasing information from, aninformation memory medium, for example, an optical disk or optical card.The present invention also relates to an optical information processingapparatus, and an inclination angle detection apparatus for detecting anangle made by a beam collected by a light collection system in anoptical information processing apparatus and an information memorymedium.

2. Description of the Related Art

Optical memory technologies which uses optical disks or optical cards ashigh density, large capacity memory media are used in progressivelywider fields, for example, in digital audio disks, video disks, documentfile disks and data files. By such optical memory technologies,information is recorded to, or reproduced from, an optical disk withsufficiently high precision and satisfactory reliability through a lightbeam which is focused to have a microscopic diameter. The performance ofa recording and reproduction apparatus using the optical memorytechnologies significantly relies on the optical system.

Exemplary basic functions of the optical head device, which is a mainpart of the optical system, are rough classified into:

(1) light collection in order to form a smallest possible light spotonly limited by the diffraction;

(2) focusing and tracking control of the optical system, andreproduction of information signals; and

(3) erasing and writing of information signals by collected light.

These functions are realized by a combination of various optical systemsand a light detector of a photoelectric conversion system.

As a first conventional example comparative to the present invention, aconventional optical head device will be described with reference toFIG. 42. FIG. 42 is a schematic view of an optical system of theconventional optical head device. In the optical head device shown inFIG. 42, focusing is performed by the non-point aberration method andtracking is performed by the push-pull method and the phase contrastingmethod.

The optical head device shown in FIG. 42 operates in the followingmanner.

Light emitted by a semiconductor laser 101 as a light source isreflected by a plane-parallel beam splitter 102 and collimated by acollimator lens 103, which is included in a light collection system. Thelight is then collected by an objective lens 104 which is also includedin the light collection system, and collected on an information lever108 of an optical disk 105, which is an information memory medium. Anactuator 107 moves the objective lens 104 and a holding device 106 inaccordance with fluctuations or decentration of the optical disk 105.

The light is then diffracted and reflected by the information layer 108of the optical disk 105 to be reflected light 105 a. The reflected light108 a is converged by the collimator lens 103. The reflected light 108 ais then provided with an non-point aberration when passing through theplane-parallel beam splitter 102. The light provided with the non-pointaberration is received by a light detector 150. The above-describedelements in the optical system shown in FIG. 42 are arranged so that,when a focal point F0 of the light from the objective lens 104 is on theinformation layer 108, a light detecting surface of the light detector150 is in the least circle of confusion of the converged light providedwith the non-point aberration.

FIG. 43A shows a pattern of a light detection area of the light detector150 and the shape of a cross section of the reflected light 108 adetected by the light detector 150. The light detector 150 includes fourlight detection areas 251 through 254. Signals obtained in accordancewith the amount of light received by the light detection areas 251through 254 are referred to herein as s1 through s4. Although anoperation circuit for generating a tracking error signal is not shown, atracking error signal TE1 is generated according to expression (1).

TE1=(s1+s4)−(s2+s3)  (1)

By the phase contrasting method, a tracking error signal TE2 is obtainedby comparing the phase of a sum signal of s1 and s3 and the phase of asum signal of s2 and s4.

A focusing error FE signal by the non-point aberration method isgenerated according to expression (2).

FE=(s1+s2)−(s2:s4)  (2)

When the information layer 108 of the optical disk 105 is distanced fromthe objective lens 104 so as to be beyond the focal point P0 of thelight from the objective lens 104, the cross section of the reflectedlight 108 a detected by the light detector 150 is as shown in FIG. 43B.When the information layer 108 of the optical disk 105 approaches theobjective lens 104 so as to be between the objective lens 104 and thefocal point F0 of the light from the objective lens 104, the crosssection of the reflected light 108 a detected b the light detector 105is as shown in FIG. 43C.

An RF signal, which is an information reproduction signal, is a sum ofthe signals s1 through s4 obtained from all the light detection areasand thus is generated according to expression (3).

RF=s1+s2+s3+s4  (3)

The conventional optical head device described above have the followingproblems.

(1) The tracking error signal is generated by a differential signalwhich indicates the difference between the signals respectively obtainedfrom the two light detection areas defined by simply equally dividingthe light detection surface (aperture) of the light detector 150 intotwo by a central line of the aperture, in such a structure, the light isincident off the track or tracking is not stably controlled when anaberration occurs due to an inclination of the objective lens 104 and/orthe optical disk 105 (tilt), or when the objective lens moves in adirection perpendicular to the tracks with respect to the optical axisin accordance with the decentration of the optical disk 105.

(2) When the focal point of the light from the objective lens 104 scansthe position off the track in which the information to be reproduced isstored, if a reproduction signal is generated by a signal indicating thedifference between the signals respectively obtained from the two lightdetection areas defined by simply equally dividing the aperture of thelight detector 150 into two by a central line of the aperture, asufficient margin with respect to the disturbance cannot be secured.

Regarding an inclination angle detection apparatus for detecting aninclination of a beam collected by a light collection system in anoptical information processing apparatus with respect to the informationmemory device, various structures have been proposed in order toaccurately read information from, and write information to, theinformation memory device.

As a second conventional example comparative to the present invention, aconventional inclination detection apparatus will be described withreference to FIG. 44. FIG. 44 is a schematic view of an inclinationdetection apparatus. The inclination detection apparatus shown in FIG.44 operates in the following manner.

A linearly polarized scattering beam 70 emitted from a semiconductorlaser 101 as a light source is collimated by a collimator lens 103 andthen is incident on a polarizing beam splitter 130. Next, the beam 70 istransmitted through the polarizing beam splitter 130 and then through a¼-wave plate 122 to be converted into a circularly polarized beam. Thecircularly polarized beam is collected on an optical disk 105 as aninformation memory medium by an objective lens 104.

FIG. 45 shows a structure of the optical disk 105. In FIG. 45, Gn−1, Gn,Gn+1, . . . each represent a guide groove. Information is stored in theguide grooves as a mark or a space. Accordingly, tracks Tn−1, Tn, Tn+1,. . . for storing information correspond to the guide grooves, Gn−1, Gn,Gn+1, . . . . Also in FIG. 45, Gp represents a space between twoadjacent guiding grooves (i.e., cycle of the grooves), and tp representsa space between two adjacent tracks (i.e., cycle of the tracks). Thevalues of Gp and tp are equal to each other.

The beam 70 which is reflected and diffracted by the optical disk 105 isagain transmitted through the objective lens 104 and then through the¼-wave plate 122 to be converted into a linearly polarized beam whichruns in a direction perpendicular to the direction of the light emittedfrom the semiconductor laser 101. The beam 70 is then entirely reflectedby the polarizing beam splitter 130 and converted into a converged beam(still indicated by reference numeral 70) by a detection lens 133. Theconverged beam 70 is transmitted to the planar polarizing plate 134 andreceived by a light detector 158. The beam 70 is provided with anon-point aberration for focusing error detection when passing throughthe planar polarizing plate 14. The beam 70 received by the lightdetector 158 is converted into an electric signal in accordance with thelight amount thereof.

In this specification, in the case where the optical disk is a RO disk,a mark indicates a pit, and a space indicates a phone part. In the casewhere the optical disk indicates a phase-change memory medium, a markindicates an amorphous portion and space indicates a crystal portion, ora mark indicates a crystal portion and a space indicates an amorphousportion. In the case where the optical disk is a magnetic memory mediuma mark indicates an upward magnetization and a space indicates adownward magnetization, or a mark indicates a downward magnetization anda space indicates an upward magnetization. Alternatively, in the casewhere the optical disk is a magnetic memory medium, a mark may indicatea rightward magnetization and a space may indicate a leftwardmagnetization, or a mark may indicate a leftward magnetization and spaceway indicate a rightward magnetization. In the case where the opticaldisk is a write-once disk such as a CD-R, a mark indicates a dye burnedarea and a space indicates a non dye burned area.

The focusing error signal and the tracking error signal are each addedto the actuator 107. The position of the objective lens 104 is adjustedso that the beam 70 emitted by the light source 101 is focused at adesired position on the optical disk 105. The methods for generating afocusing error signal and a tracking error signal are well known andthus will not be described here.

FIG. 46 shows a signal processing section 703 including the lightdetector 158. The electric signal from the light detector 158 is inputto the signal processing section 703. As shown in FIG. 46, the lightdetector 158 includes four light detection sections 158A, 158B, 158C and158D. The signals from the light detection sections 158A through 158Dare respectively current/voltage converted by current/voltage converters855 through 858. The signals from the current/voltage converters 855through 858 are sent to an operation section 871 for a differentialoperation. The signal from the operation section 871 is output from aterminal 814. The signal from the terminal 814 is an inclinationdetection signal.

In the case where an inclination is detected by the above-describedconventional inclination detection apparatus utilizing that eclipse ofthe beam 70 reflected by the optical disk 105 occurs by the aperturediaphragm of the objective lens 104, the detection sensitivity reducesas the numerical aperture of the objective lens 104 increases. Recently,a structure has been proposed in which the numerical aperture of thelight collection system is 0.6 and the thickness of the informationmemory medium is 0.6 mm in order to increase the information which canbe stored in one information memory medium. In such a structure, a mereabout 0.6 degree change in the angle made by the beam collected by theobjective lens and the information memory medium significantly changesthe jitter characteristics of the information read from the informativememory medium. In the case where an inclination servo for compensatingfor the change in the angle made by the beam collected by the objectivelens is introduced, the inclination detection apparatus needs to detectthe inclination with an error of 0.5 degrees or less. However, in theconventional inclination detection apparatus, when the numericalaperture of the objective lens is 0.6, even if the inclination isactually, for example, 0.5 degrees, the inclination detection signalchanges only by about 2%. Thus, it is difficult to precisely detect theinclination of 0.5 degrees or less.

As a third example comparative to the present invention, anotherconventional optical head device will be described with reference toFIG. 47.

A linearly polarized scattering beam 70 emitted by a semiconductor laser101 as a light source is collimated by a collimator lens 103 and then isincident on a polarizing beam splitter 130. The beam 70 is transmittedthrough the polarizing beam splitter 130 and then through a ¼-wave plate122 to be converted into a circularly polarized beam. The circularlypolarized beam is collected on an optical disk 105 by an objective lens104. The beam 70 reflected and diffracted by the optical disk 105 isagain transmitted through the objective lens 104 and then through the¼-wave plate 122 to be converted into a linearly polarized beam whichtravels in a direction perpendicular to the direction of the lightemitted from the semiconductor laser 101. The beam 70 is then entirelyreflected by the polarizing beam splitter 130 and converted into aconverged beam (still indicated by reference numeral 70) by a detectionlens 133. The converted beam 70 is transmitted to the planar polarizingplate 134 and received by a light detector 158. The beam 70 is providedwith a non-point aberration for focusing error detection when passingthrough the planar polarizing plate 134. The beam 70 received by thelight detector 158 is converted into an electric signal in accordancewith the light amount thereof.

FIG. 48 shows a signal processing section 705 including the lightdetector 158. The electric signal from the light detector 158 is inputto the signal processing section 705. As shown in FIG. 48, the lightdetector 158 includes four light detection sections 158A, 158B, 158C and158D. The signals from the light detection sections 158A through 158Dare respectively current/voltage converted by current/voltage converters851 through 854. The signals from the current/voltage converters 851 and854 are added together by an addition section 891, the signals from thecurrent/voltage converters 852 and 853 are added together by an additionsection 892, the signals from the current/voltage converters 851 and 853are added together by an addition section 893, and the signals from thecurrent/voltage converters 852 and 854 are added together by an additionsection 894. The signals from the adding sections 891 and 892 are sentto an operation section 871 for a differential operation, and thesignals from the adding sections 893 and 894 and sent to an operationsection 872 for a differential operation. The signal from the operationsection 871 is output from a terminal 811, and the signal from theoperation section 872 is output from a terminal 812. The signal outputfrom the terminal 811 is a tracking error signal, and the signal outputfrom the terminal 812 is a focusing error signal. The focusing errorsignal is generated by a well known method referred to as the “non-pointaberration method”, and the tracking error signal is generated by a wellknown method referred to as the “push-pull” method. The focusing errorsignal and the tracking error signal are respectively added to anactuator 107 for focusing control and another actuator 107 for trackingcontrol. The position of the objective lens 104 is adjusted so that thebeam 70 from the semiconductor laser 101 is focused at a desirableposition on the optical disk 105.

FIG. 49 shows a structure of the optical disk 105 (FIG. 47). In FIG. 49,Gn−1, Gn, Gn+1, . . . each represent a guide groove for allowingtracking error signal detection. Information is stored in and betweenthe guide grooves as a mark or a space. Where a space between twoadjacent guiding grooves is Gp and a space between two adjacent tracksis tp, Gp=2•tp.

In the optical head device described as the third example, the followingconditions, for example, are adopted in order to store a great amount ofinformation in the optical disk 105. The wavelength λ of the beam 70from the semiconductor laser 101 as the light source is 650 nm, thenumerical aperture NA of the objective lens 104 is 0.6, the thickness tof the optical disk 105 is 0.6 mm, the distance Gp between centers oftwo adjacent guiding grooves is 1.48 μm, and the distance tp betweencenters of two adjacent tracks is 0.74 μm. When the angle made by thebeam 70 collected by the objective lens 104 and the optical disk 105 isa proper angle, the tracking error signal zero-crosses when the centerof the guiding groove is irradiated by the beam 70 collected by theobjective lens 104. However, when the angle made by the beam 70collected by the objective lens 104 and the optical disk 105 is not aproper angle, the tracking error signal does not zero-cross when thecenter of the guiding groove is irradiated by the beam 70 collected bythe objective lens 104. At this point, the tracking error signal ishardly offset but is phase-shifted. Such a phase shift can be a cause ofan off-track. For example, when the phase shift is about 0.5 degrees, a0.1 μm off-track is caused. When the off-track is caused, theinformation stored in the optical disk cannot be accurately read orerased.

SUMMARY OF THE INVENTION

According to one aspect of the invention, an optical head deviceincludes a light source for emitting at least one of a coherent beam anda quasi-monochromatic beam; a collection optical system for collectingthe beam emitted by the light source to an information memory mediumhaving a track which has at least one mark and at least one space; alight detector having a plurality of detection areas for receiving thebeam reflected by the information memory medium and outputting a signalin accordance with a light amount of the beam received; and a trackingerror signal generator for receiving the signals output from the lightdetector and generating a tracking error signal based on the signals.The tracking error signal generator reduces a difference between a firstsignal amplitude and a second signal amplitude. The first signalamplitude is an absolute value of a difference between a first signallevel and a second signal level. The second signal amplitude is anabsolute value of a difference between the first signal level and athird signal level. The first signal level is a value of the trackingerror signal obtained when the beam emitted by the light source isradiated to a center of the track. The second signal level is a maximumvalue of the tracking error signal obtained when the information memorymedium is scanned by the beam emitted by the light source in a directionperpendicular to the track. The third signal level is a minimum value ofthe tracking error signal obtained when the information memory medium isscanned by the beam emitted by the light source in a directionperpendicular to the track.

According to another aspect of the invention, an optical head deviceincludes a light source for emitting at least one of a coherent beam anda quasi-monochromatic beam; a collection optical system for collectingthe beam emitted by the light source to an information memory mediumhaving at least one track, at least one mark and at least one space; alight detector having a plurality of detection areas for receiving thebeam reflected by the information memory medium and outputting a signalin accordance with a light amount of the beam received; a tracking errorsignal generator for receiving the signals output from the lightdetector and generating a tracking error signal based on the signals.The tracking error generator subtracts, from the tracking error signal,a component of the signal obtained from an overlapping area. In the casewhere an aperture of the collection optical system is a circle having aradius of 1, the overlapping area is an area where two circles overlap,the circles each having a radius of 1 and being centered around a pointwhich is λ/NA•Gp) away, in a direction perpendicular to the track, froma center of the aperture, where λ is the wavelength of the beam emittedby the light source. NA is the numerical aperture of the collectionoptical system, Gp is the distance between centers of two adjacenttracks of the information memory medium, and λ/(NA•Gp)<1.

According to still another aspect of the invention, an optical headdevice includes a light source for emitting at least one of a coherentbeam and a quasi-monochromatic beam; a collection optical system forcollecting the beam emitted by the light source to an information memorymedium having at least one track, at least one mark and at least onespace; a light detector having a plurality of detection areas forreceiving the beam reflected by the information memory medium andoutputting a signal in accordance with a light amount or the baemreceived; a tracking error signal generator for receiving the signalsoutput from the light detector and generating a tracking error signalbased on the signals; and a light division element for dividing anoverlapping area of the reflected beam and the vicinity thereof so as tobe received by the light detector. The vicinity of the overlapping arearefers to an area which is distanced from the overlapping area by aprescribed distance. In the case where an aperture of the collectionoptical system is a circle having a radius of 1, the overlapping area isan area where two circles overlap, the circles each having a radius of 1and being centered around a point which is λ/NA•Gp) away, in a directionperpendicular to the track, from a center of the aperture, where λ isthe wavelength of the beam emitted by the light source, NA is thenumerical aperture of the collection optical system, Gp is the distancebetween centers of two adjacent tracks of the information memory medium,and λ/(NA•GP)<1.

In one embodiment of the invention, the tracking error signal generatorgenerates a tracking error signal using a signal obtained from thedetector area which receives a beam in an area excluding the overlappingarea, the beam being included in the reflected baeam.

In one embodiment of the invention, the light division element includesat least two division lines which are substantially parallel to thetracks. The at least two division lines are arranged so as to sandwichthe overlapping area of the reflected beam therebetween, and thetracking error signal generator generates a tracking error signal basedon an operation of a signal obtained from the detection area whichreceives a beam incident on an area outside the at least two divisionlines, the beam being included in the reflected beam.

In one embodiment of the invention, the tracking error signal generatorcorrects a tracking error signal using a signal obtained from thedetection area which receives a beam in the overlapping area and thevicinity thereof, the beam being included in the reflected beam.

In one embodiment of the invention, the light division element includesdivision lines in the number of N which are substantially parallel tothe tangent to the tracks, wherein N is an odd integer of 3 or more. Thetwo of the division lines are arranged so as to sandwich the overlappingarea of the reflected beam therebetween. The remaining division linesare arranged between the two of the division lines. The tracking errorsignal generator generates a tracing error signal using signals obtainedfrom the detection area which receives a beam incident on a first areaand a second area which are outside the two of the division lines andexclude the overlapping areas, the beam being included in the reflectedbeam. The tracking error signal generator further generates a correctionsignal by alternately inverting the polarity of signals obtained fromthe detection area which receives a beam incident on an even number ofareas sandwiched between the two of the division lines, the beam beingincluded in the reflected beam, and then adding together the signalsobtained from the detection area. The tracking error signal generatorthen adds the tracking error signal and the correction signal orsubtracts the correction signal from the tracking error signal.

In one embodiment of the invention, the light division element includesdivision lines in the number of N which are substantially parallel tothe tangent to the tracks, wherein N is an odd integer of 3 or more. Thetwo of the division lines are arranged an as to sandwich the overlappingarea therebetween. The remaining division lines are arranged between thetwo of the division lines. The tracking error generator generates acorrection signal by multiplying a value of each of the signals with aprescribed value, the signals being obtained from the detection areawhich receives a beam incident on an even number of areas sandwichedbetween the two of the division lines, the beam being included in thereflected beam, and then alternately inverting the polarity of theresultant signals, and adding together those signals. The tracking errorsignal generator than adds the tracking error signal and the correctionsignal or subtracts the correction signal from the tracking errorsignal.

In one embodiment of the invention, the light division element is aholographic element.

In one embodiment of the invention, the light division element isintegral with a collection optical system.

In one embodiment of the invention, the light division element is adivision line of the light detector.

According to still another aspect of the invention, an optical headdevice includes a light source for emitting at least one of a coherentbeam and a quasi-monochromatic beam; a collection optical system forcollecting the beam emitted by the light source to an information memorymedium having at least one track, at least one mark and at least onespace; a light detector having a plurality of detection areas forreceiving the beam reflected by the information memory medium andoutputting a signal in accordance with a light amount of the beamreceived; a tracking error signal generator for receiving the signalsoutput from the light detector and generating a tracking error signalbased on the signals; and a light reduction element provided on a beampath for reducing the light transmittance of the overlapping area andthe vicinity thereof. In the case where an aperture of the collectionoptical system is a circle having a radius of 1, the overlapping area isan area where two circles overlap, the circles each having a radius of 1and being centered around a point which is λ/(NA•Gp) away, in adirection perpendicular to the track, from a center of the aperture,where λ is the wavelength of the beam emitted by the light source, NA isthe numerical aperture of the collection optical system, Gp is thedistance between centers of two adjacent tracks of the informationmemory medium, and λ/(NA•Gp)<1.

In one embodiment of the invention, the light reduction element isintegral with the collection optical system.

In one embodiment of the invention, the light reduction element is aholographic element.

According to still another aspect of the invention, an optical headdevice includes a light source for emitting at least one of a coherentbeam and a quasi-monochromatic beam; an optical element for receivingthe beam emitted by the light source and dividing the beam into firstbeam and a second beam an effective numerical aperture of the collectionoptical system with respect to the first been being different from aneffective numerical aperture of the collection optical system withrespect to the second beam; a collection optical system for receivingthe first beam and the second beam and converging the first and secondbeams into a microscopic spot on an information memory medium; a beambranching element for receiving the beam diffracted and/or reflected bythe information memory medium and branching the beam; a light detectorfor receiving the branched beam and outputting a signal in accordancewith a light amount of the beam received; a signal processing sectionfor receiving the signal from the light detector and performing anoperation of the signal; a driving section for determining relativepositions of the collection optical system and the information memorymedium based on the signal output from the signal processing section;and a tracking error signal generator for generating a tracking errorsignal using the first or second beam with respect to which theeffective numerical aperture of the collection optical system issmaller.

In one embodiment of the invention, the information memory mediumincludes marks or prescribed grooves for realizing detection of thetracking error signal, where Gp is the cycle of the marks or grooves,and NA is the numerical aperture of the collection optical system, thefirst beam has a wavelength λ represented by Gp<λ/NA, and the secondbeam has a wavelength λ represented by Gp<λ/NA, and the tracking errorsignal generator generates a tracking error signal based on the secondbeam. In this specification, the cycle of the grooves refers to thedistance between the center of one groove end the center of a grooveadjacent thereto.

In one embodiment of the invention, an optical axis of the first beam issubstantially coincident with an optical axis of the second beam.

In one embodiment of the invention the optical element is a polarizationfilter.

In one embodiment of the invention, the optical element is integral withthe collection optical system.

According to still another aspect of the invention, an inclinationdetection apparatus includes a light source for emitting at least one ofa coherent beam and a quasi-monochromatic beam; a collection opticalsystem for receiving the beam emitted by the light source and convergingthe beam into a microscopic spot on an information memory medium; a beambranching element for receiving the beam diffracted and/or reflected bythe information memory medium and branching the beams a light detectorfor receiving the branched beam and outputting a signal in accordancewith a light amount of the beam received; a signal processing sectionfor receiving the signal from the light detector and performing anoperation of the signal; and a driving section for performing focusingcontrol and tracking control to determine relative positions of thecollection optical system and the information memory medium. The lightdetector includes a plurality of detection areas. The information memorymedium has a first pattern area including a mark and a space and asecond pattern area including prescribed grooves. The first pattern areaand the second pattern area are alternately arranged on the informationmemory medium. The signal processing section detects an angle made bythe beam collected by the collection optical system and the informationmemory medium, using a signal obtained by the light detector when one ofthe first pattern area and the second pattern area is irradiated by thebeam collected by the collection optical system.

In one embodiment of the invention, in the case where the mark and thespace in the first pattern area are irradiated by the beam collected bythe collection optical system, tracking control is performed using thesignal obtained by the light detector. In the case where the secondpattern area is irradiated by the beam collected by the collectionoptical system, the angle made by the beam collected by the collectionoptical system and the information memory medium is detected using asignal obtained by the light detector.

In one embodiment of the invention, in the case where the second patternarea is irradiated by the beam collected by the collection opticalsystem, tracking control is performed using the signal obtained by thelight detector. In the case where the first pattern area is irradiatedby the beam collected by the collection optical system, the angle madeby the beam collected by the collection optical system and theinformation memory medium is detected using a signal obtained by thelight detector.

In one embodiment of the invention, in the case where the mark and thespace of the first pattern area are irradiated by the beam collected bythe collection optical system, the angle made by the beam collected bythe collection optical system and the information memory medium isdetected using a signal obtained by the light detector.

In one embodiment of the invention, the inclination detection apparatushas the relationship of NA<λ/Gp where Gp is the cycle of marks in thefirst pattern area or the cycle of the grooves in the second patternarea, λ is the wavelength of the beam emitted by the light source, andNA is the numerical aperture of a part of the collection optical systemfacing the information memory medium.

According to still another aspect of the invention, an opticalinformation processing apparatus includes a light source for omitting atleast one of a coherent beam and a quasi-monochromatic beam; acollection optical system for receiving the beam emitted by the lightsource and converging the beam into a microscopic spot on an informationmemory medium; a beam branching element for receiving the beamdiffracted and/or reflected by the information memory medium andbranching the beam; a light detector for receiving the branched beam andoutputting a signal in accordance with a light amount of the beamreceived; a signal processing section for receiving the signal from thelight detector and performing an operation of the signal; a firstdriving section for performing focusing control and tracking control todetermine relative positions of the collection optical system and theinformation memory medium; and a second driving section for changing theangle made by the beam collected by the collection optical system andthe information memory medium. The light detector includes a pluralityof detection areas. The information memory medium has patterns orprescribed grooves for generating a tracking error signal. NA>λ/Gp whereGp is the cycle of patterns or grooves, λ is the wavelength of the beamemitted by the light source, and NA is the numerical aperture of a partof the collection optical system facing the information memory medium.

According to still another aspect of the invention, an opticalinformation prossing apparatus includes a light source for emitting atleast one of a coherent beam and a quasi-monochromatic beam; acollection optical system for receiving the beam emitted by the lightsource and converging the beam into a microscopic spot on an informationmemory medium; a beam branching element for receiving the beamdiffracted and/or reflected by the information memory medium andbranching the beam; a light detector for receiving the branched beam andoutputting a signal in accordance with a light amount of the beamreceived; a signal processing section for receiving the signal from thelight detector and performing an operation of the signal; a firstdriving section for performing focusing control and tracking control todetermine relative positions of the collection optical system and theinformation memory medium; and a second driving section for changing theangle made by the beam collected by the collection optical system andthe information memory medium. The light detector includes a pluralityof detection areas. The information memory medium has a first patternarea including a mark and a space and a second pattern area includingprescribed grooves. The first pattern area and the second pattern areaare alternatively arranged on the information memory medium. The signalprocessing section detects an angle made by the beam collected by thecollection optical system and the information memory medium, using asignal obtained by the light detector, and also generates a signal fordriving the second driving section, when one of the first pattern areaand the second pattern area is irradiated by the beam collected by thecollection optical system.

In one embodiment of the invention, in the case where the mark in thefirst pattern area is irradiated by the beam collected by the collectionoptical system, tracking control is performed using the signal obtainedby the light detector. In the case where the second pattern area isirradiated by the beam collected by the collection optical system, theangle made by the beam collected by the collection optical system andthe information memory medium is detected using a signal obtained by thelight detector.

In one embodiment of the invention, in the case where the second patternarea is irradiated by the beam collected by the collection opticalsystem, tracking control is performed using the signal obtained by thelight detector. In the case where the first pattern area is irradiatedby the beam collected by the collection optical system, the angle madeby the been collected by the collection optical system and theinformation memory medium is detected using a signal obtained by thelight detector.

In one embodiment of the invention, in the case where the mark and thespace of the first pattern area are irradiated by the beam collected bythe collection optical system, the angle made by the beam collected bythe collection optical system and the information memory medium isdetected using a signal obtained by the light detector.

In one embodiment of the invention, the optical information prossingapparatus has the relationship of NA>λ/Gp where Gp is the cyclic ofmarks in the first pattern area or the cycle of the grooves in thesecond pattern area. λ is the wavelength of the beam emitted by thelight source, and NA is the numerical aperture of a part of thecollection optical system facing the information memory medium.

According to still another aspect of the invention, an optical headdevice includes a light source for emitting at least one of a coherentbeam and a quasi-monochromatic beam; a collection optical system forcollecting the beam emitted by the light source to an information memorymedium having a track which has at least one mark and at least one spaceselectively arranged; a light detector for receiving the beam reflectedby the information memory medium and outputting a signal in accordancewith a light amount of the beam received; a light division element fordividing the beam reflected by the information memory medium so as to bereceived by the light detector; an information reproduction signalgenerator for generating an information reproduction signal forreproducing information stored in the track, based on a signalindicating the difference between the beams incident on a first area anda second area defined by a division line of the light division element;and a changing element for changing a region included in the first area,a region included in the second area, or a region included in both thefirst area and the second area in accordance with the positionalrelationship between the light collection point of the light from thecollection optical system and the track.

In one embodiment of the invention, in the case where the beam has asubstantially circular cross section having a radius of 1 on the lightdivision element, the light division element is divided into three areasby a first division line which is substantially parallel to the tangentto the track and is distanced from the center of the substantiallycircular cross section by a prescribed distance d, and a second divisionline which is substantially parallel to the tangent to the track and isdistanced from the center of the substantially circular cross section bythe prescribed distance d in an opposite direction to the first divisionline; the area which is outside the first division line and thusexcludes the center of the substantially circular cross section may bedefined as area A, the area sandwiched by the first division line andthe second division line may be defined as area B, and the area which isoutside the second division line and thus excludes the center of thesubstantially circular cross section may be defined as area C. When thelight collection point from the collection optical system is at a firstposition on the information memory medium which is distanced in onedirection from the track by a prescribed distance, the informationreproduction signal generator generates an information reproductionsignal for reproducing information stored in the information memorymedium, with the area A being the first area and a sum of the areas Band C being the second area. When the light collection point from thecollection optical system is at a second position on the informationmemory medium which is distanced from the track by the prescribeddistance in an opposite direction to the first position, the informationreproduction signal generator generates an information reproductionsignal for reproducing information stored in the track, with a sum ofthe area A and B being the first area and the area C being the secondarea.

In one embodiment of the invention, where the beam has a substantiallycircular cross section having a radius of 1 on the light divisionelement, the light division element is divided into four areas by afirst division line which is substantially parallel to the tangent tothe track and is distanced from the center of the substantially circularcross section by a prescribed distance d, a second division line whichis substantially parallel to the tangent to the track and is distancedfrom the center of the substantially circular cross section by theproscribed distance d in an opposite direction to the first divisionline, and a third division line which is substantially parallel to thetangent to the track and runs through the center of the substantiallycircular cross section; the area which is outside the first divisionline and thus excludes the center of the substantially circular crosssection may be defined as area A, the area sandwiched by the firstdivision line and the third division line may be defined as area D, thearea sandwiched by the first division line and the second division linemay be defined as area C, and the area which is outside the seconddivision line and thus excludes the center of the substantially circularcross section may be defined as area D. When the light collection pointfrom the collection optical system is at a first position on theinformation memory medium which is distanced in one direction from thetrack by a prescribed distance, the information reproduction signalgenerator generates on information reproduction signal for reproducinginformation stored in the information memory medium, with the area Abeing the first area and the area C and D being the second area. Whenthe light collection point from the collection optical system is at asecond position on the information memory medium which is distanced fromthe track by the prescribed distance in an opposite direction to thefirst position, the information reproduction signal generator generatesan information reproduction signal for reproducing information stored inthe track, with the areas A and B being the first area and the area Dbeing the second area.

According to still another aspect of the invention, an optical headdevice includes a light source for omitting at least one of a coherentbeam and a quasi-monochromatic beam; a collection optical system forcollecting the beam emitted by the light source to an information memorymedium having a track which has at least one mark and at least one spaceselectively arranged; a light detector having a plurality of detectionareas for receiving the beam reflected by the information memory mediumand outputting a signal in accordance with a light amount of the beamreceived; a light division element for dividing the beam reflected bythe information memory medium so as to be received by the lightdetector; and an information reproduction signal generation fungenerating an information reproduction signal for reproducinginformation stored in the track, based on a signal indicating thedifference between the beams incident on a first area and a second areadefined by a division line of the light division element. In the casewhere the beam has a substantially circular cross section having aradius of 1 on the light division element, the light division element isdivided into three areas by a first division line which is substantiallyparallel to the tangent to the track and is distanced from the center ofthe substantially circular cross section by a prescribed distance d, anda second division line which is substantially parallel to the tangent tothe track and is distanced from the center of the substantially circularcross section by the prescribed distance d in an opposite direction tothe first division line, one of the areas excluding the center of thesubstantially circular cross section may be defined as the first area,and another area excluding the center of the substantially circularcross section may be defined as the second area. When the lightcollection point from the collection optical system is at a position onthe information memory medium which is distanced from the track by aprescribed distance, the information reproduction signal generatorgenerates an information reproduction signal for reproducing informationstored in the track.

According to still another aspect of the invention, an optical headdevice includes a light source for emitting one of a coherent beam or aquasi-monochromatic beam; a collection optical system for collecting thebeam emitted by the light source to an information memory medium havinga track which has at least one mark and at least one space selectivelyarranged; a light detector having a plurality of detection areas forreceiving the beam reflected by the information memory medium andoutputting a signal in accordance with a light amount of the beamreceived; a light division element for dividing the beam reflected bythe information memory medium so as to be received by the lightdetector; and a information reproduction signal generator for generatingan information reproduction signal for reproducing information stored inthe track, based on a signal indicating the difference between the beamsincident on a first area and a second area defined by a division line ofthe light division element. In the case where the beam has asubstantially circular cross section having a radius of 1 on the lightdivision element, the light division element is divided into four areasby a first division line which is substantially parallel to the tangentto the track and is distanced from the center of the substantiallycircular cross section by a prescribed distance d, a second divisionline which is substantially parallel to the tangent to the track and isdistanced from the center of the substantially circular cross section bythe prescribed distance d in an opposite direction to the first divisionline, and a third division line which is substantially parallel to thetangent to the track and runs through the center of the substantiallycircular cross section, a sum of the area which is outside the firstdivision line and thus excludes the center of the substantially circularcross section and the area sandwiched by the third division line and thesecond division line may be defined an the first area, and a sum of thearea sandwiched by the first division line and the third division lineand the area which is outside the second division line and thus excludesthe center of the substantially circular cross section may be defined asa second area. When the light collection point from the collectionoptical system is at a position on the information memory medium whichis distanced from the track by a prescribed distance, the informationreproduction signal generator generates an information reproductionsignal for reproducing information stored in the track.

In one embodiment of the invention, in the case where the beam has asubstantially circular cross section having a radius of 1 on the lightdivision element, the distance d between the center of the substantiallycircular cross section and each of the first division line end thesecond division line is 0.1 or more and 0.3 or less.

According to still another aspect of the invention, an optical headdevice includes a light source for emitting at least one of a coherentbeam and a quasi-monochromatic beam; a collection optical system forcollecting the beam emitted by the light source to an information memorymedium having tracks having a mark and a space selectively arranged ortracks having prescribed grooves; a light detector having a plurality ofdetection areas for receiving the beam reflected by the informationmemory medium and outputting a signal in accordance with a light amountof the beam received; a tracking error signal generator for receivingthe signals from the light detector and generating a tracking errorsignal based on the signals received; and a light division element fordividing the beam reflected by the information memory medium so as to bereceived by the light detector. Where λ is the wavelength of the beamemitted by the light source, NA is the numerical aperture of thecollection optical system, Gp is the distance between centers of theadjacent tracks of the information memory medium, λ/(NA•Gp)≧1, and thebeam has a substantially circular cross section having a radius of 1 onthe light division element. The light division element has at least fivedivision lines which are substantially parallel to the tangent to thetracks. Where the division line running through the center of thesubstantially circular cross section is a first division line, twodivision lines which are distanced from the first division line by adistance of about 0.1 in the two opposite directions are a seconddivision line and a third division line, and two division lines whichare distanced from two ends of the cross section by a distance of about0.1 are a fourth division line and a fifth division line. The trackingerror signal generator generates the tracking error signal byalternately inverting the polarity of the signals obtained in accordancewith the beams incident on six areas defined by the five division linesand adding together those signals.

According to still another aspect of the invention, an optical headdevice includes a light source for emitting at least one of a coherentbeam and a quasi-monochromatic beam; a collection optical system forcollecting the beam emitted by the light source to an information memorymedium having tracks having a mark and a space selectively arranged ortracks having prescribed grooves; a light detector having a plurality ofdetection areas for receiving the beam reflected by the informationmemory medium and outputting a signal in accordance with a light amountof the beam received; a tracking error signal generator for receivingthe signals from the light detector and generating a tracking errorsignal based on the signals received; and a light division element fordividing the beam reflected by the information memory medium so as to bereceived by the light detector. Where λ is the wavelength of the beamemitted by the light source, NA is the numerical aperture of thecollection optical system, Gp is the distance between centers of twoadjacent tracks of the information memory medium, λ/(NA•Gp)≧1, and anaperture of the collection optical system is a circle having a radiusof 1. The light division element has division lines in the number of Nwhich are substantially parallel to the tangent to the tracks where N isan odd integer of 3 or more. The two of the division lines arepositioned within a width of about 0.6 from the center of the apertureof the collection optical system. The remaining division lines arepositioned between the two division lines at an equal interval. Thetracking error signal generator generates the tracking error signalusing signals obtained from the areas which are outside the two divisionlines and thus exclude the center of the substantially circular crosssection. The tracking error signal generator generates a correctionsignal by alternately inverting the polarity of signals obtained from aneven number of areas sandwiched by the two division lines and addingtogether those signals. The tracking error signal generator adds thetracking error signal and the correction signal or subtracts thecorrection signal from the tracking error signal.

In one embodiment of the invention, the line division element is adiffraction element.

In one embodiment of the invention, the line division element is adivision line of the light detector

According to still another aspect of the invention, an optical headdevice includes a light source for emitting at least one of a coherentbeam and a quasi-monochromatic beam; a collection optiosl system forcollecting the beam emitted by the light source to an information memorymedium having tracks having a mark and a space selectively arranged ortracks having prescribed grooves; a diffraction element for receiving abeam diffracted by the information memory medium and generating adiffraction beam; and a light detector having a plurality of detectionareas for receiving the beam diffracted by the diffraction element andoutputting a signal in accordance with a light amount of the beamreceived. The diffraction element includes a plurality of areas. Adiffraction beam of a desired order generated by an area group Aincluded in the plurality of areas form a first spherical wave. Adiffraction beam of a desired order generated by an area group Bincluded in the plurality of areas but excluded from the area group Aform a second spherical wave, which has a light collection point fartherthan the light collection point of the first spherical waive withrespect to the diffraction element. A focusing error signal generator isprovided for generating a focusing error signal based on the differencebetween the cross sections of the first spherical wave and the secondspherical wave on the light detector. The diffraction element has atleast one division line perpendicular to the tangent to the tracks.Either one of portions sandwiching the at least one division line isincluded in the area group A and the other portion is included in thearea group B.

According to still another aspect of the invention, an optical headdevice includes a light source for omitting at least one of a coherentbeam and a quasi-monochromatic beam; a collection optical system forcollecting the beam emitted by the light source to an information memorymedium tracks having a mark and a space selectively arranged or trackshaving prescribed grooves; a diffraction element for receiving a beamdiffracted by the information memory medium and generating a diffractionbeam; and a light detector having a plurality of detection areas forreceiving the beam diffracted by the diffraction element and outputtinga signal in accordance with a light amount of the beam received. Thediffraction element includes a plurality of areas. A diffraction beam ofa desired order generate by an area group A included in the plurality ofareas form a first spherical wave. A diffraction beam of a desired ordergenerated by an area group B included in the plurality of areas butexcluded from the area group A form a second spherical wave, which has alight collection point farther than the light collection point of thefirst spherical wave with respect to the diffraction element. A focusingerror signal generator is provided for generating a focusing errorsignal based on the difference between the cross sections of the firstspherical wave and the second spherical wave on the light detector. Thediffraction element has a diffraction area which it larger than the areacorresponding to an aperture of the collection optical system. Thediffraction element has a first division line and a second division lineinterposing the aperture, the first division line and the seconddivision line being parallel to the tangent to the tracks and in contactwith an outer periphery of the aperture. Either one of portionssandwiching the first division line or the second division line isincluded in the area group A and the other portion is included in thearea group B.

In one embodiment of the invention, the diffraction element is integralwith the collection optical system.

According to still another aspect of the invention, a method forprocessing optical information includes the steps of emitting at leastone of a coherent beam and a quasi-monochromatic beam; collecting thebeam emitted by the light source to tin information memory medium havingat least one track, at least one mark and at least one space; receivingthe beam reflected by the information memory medium by a plurality ofdetection areas and outputting a signal in accordance with a lightamount of the beam received; and receiving the signals output from theplurality of detection areas and generating a tracking error signalbased an the signals. The step of generating a tracking error signalincludes the step of subtracting a component of the signal obtained froman overlapping area from the tracking error signal. In the case where anaperture of a collection optical system for collecting the beam is acircle having a radius of 1, the overlapping area is an area where twocircles overlap, the circles each having a radius of 1 and beingcentered around a point which is λ/NA.Gp) away, in a directionperpendicular to the track, from a center of the aperture of thecollection optical system, and where λ is the wavelength of the emittedbeam, NA is the numerical aperture of the collection optical system, andGp is the distance between centers of two adjacent tracks of theinformation memory medium, λ/(NA.Gp)<1.

According to still another aspect of the invention, a method forprocessing optical information includes the steps of emitting at leastone of a coherent beam and a quasi-monochromatic beam; receiving thebeam emitted by the light source and dividing the beam into a first beamand a second beam; receiving the first beam and the second beam andcollecting the first beam and the second beam into a microscopic spot onan information memory medium, an effective numerical aperture of thecollection optical system for collecting the beams with respect to thefirst beam being different from an effective numerical aperture of thecollection optical system with respect to the second beam; receiving thebeam diffracted and/or reflected by the information memory medium andbranching the beam; receiving the branched beam and outputting a signalin accordance with a light amount of the beam received; receiving theoutput signal and performing an operation of the signal; determiningrelative positions of the collection optical system and the informationmemory medium based on the signal obtained as a result of the operation;and generating a tracking error signal using the first or second beamwith respect to which the effective numerical aperture of the collectionoptical system is smaller.

According to still another aspect of the invention, a method forprocessing optical information includes the steps of emitting at leastone of a coherent beam and a quasi-monochromatic beam: receiving thebeam emitted by the light source and converging the beam into amicroscopic spot on an information memory medium; receiving the beamdiffracted and/or reflected by the information memory medium andbranching the beam; receiving the branched beam by a plurality ofdetection areas and outputting a signal in accordance with a lightamount of the beam received; receiving the output signal and performingan operation of the signal; performing focusing control and trackingcontrol to determine relative positions of a collection optical systemfor converging the beam and the information memory medium; and changingthe angle made by the beam converged by the collection optical systemand the information memory medium. The information memory medium has,patterns or prescribed grooves for generating a tracking error signal,and NA>λ/Gy where Gp is the cycle of the patterns or grooves, λ is thewavelength of the emitted beam, and NA is the numerical aperture of apart of the converging system facing the information memory medium.

According to still another aspect of the invention, a method forprocessing optical information includes the steps of emitting at leastone of a coherent beam and a quasi-monochromatic beam; collecting thebeam emitted by the light source to an information memory medium havinga track having a mark and a space selectively arranged; receiving thebeam reflected by the information memory medium by a plurality ofdetection areas and outputting a signal in accordance with a lightamount of the beam received; dividing the light reflected by theinformation memory medium; generating an information reproduction signalfor reproducing information stored in the track based on a signalindicating the difference between a signal obtained in accordance withthe beams incident on a first area and a second area obtained as aresult of dividing the light; and changing a region included in thefirst area, a region included in the second area, or a region includedin both the first area and the second area, in accordance with thepositional relationship between the light collection point of the lightfrom a collection optical system for collecting the beam and the track.

According to still another aspect of the invention, a method forprocessing optical information includes the steps of emitting at leastone of a coherent beam and a quasi-monochromatic beam; collecting thebeam emitted by the light source to an information memory medium havingtracks having a mark and a space selectively arranged or tracks havingprescribed grooves; receiving the beam reflected by the informationmemory medium by a detection area and outputting a signal in accordancewith a light amount of the beam received; receiving the output signaland generating a tracking error signal based on the signal received; anddividing the light reflected by the information memory medium. Where λis the wavelength of the emitted beam, NA is the numerical aperture of acollection optical system for collecting the beam, Gp is the distancebetween centers of two adjacent tracks of the information memory medium,λ/(NA.Cp)≧1, and the beam has a substantially circular cross sectionhaving a radius of 1, the light division element has at least fivedivision lines which are substantially parallel to the tangent to thetracks; the division line running through the center of thesubstantially circular cross section may be defined as a first divisionline, two division lines which are distanced from the first divisionline by a distance of about 0.1 in two opposite directions may bedefined as a second division line and a third division line, and twodivision lines which are distanced from two ends of the cross section bya distance of about 0.1 may be defined as a fourth division line and afifth division line. The method further includes the step of generatingthe tracking error signal, by alternately inverting the polarity of thesignals obtained in accordance with the beams incident on six areasdefined by the five division lines and adding together those signals.

According to still another aspect of the invention a method forprocessing optical information includes the steps of emitting at leastone of a coherent beam and a quasi-monochromatic beam; collecting thebeam emitted by the light source to an information memory medium havingtracks having a mark and a space selectively arranged or tracks havingprescribed grooves; receiving the beam reflected by the informationmemory medium by a plurality of areas and generating a diffraction beam;receiving the diffracted beam by a plurality of detection areas andoutputting a signal in accordance with a light amount of the beamreceived; and generating a focusing error signal based on the differencebetween the size of the cross section of a first spherical wave and thesize of the cross section of a second spherical wave. The firstspherical wave corresponds to a diffraction beam of a desired ordergenerated by an area group A included in the plurality of areas. Thesecond spherical wave corresponds to a diffraction beam of a desiredorder generated by an area group B included in the plurality of areasbut excluded from the area group A. Either one of portions sandwichingthe at least one division line is included in the area group A end theother portion is included in the area group B.

Thus, the invention described herein makes possible the advantages of(1) providing an optical head device having stable servo characteristicsand thus realizes stable formation of marks at appropriate positions onor in the vicinity of the tracks for information recording, and alsorealizes correct information reproduction and stable informationrecording and erasure with a sufficiently low error ratio; (2) providingan inclination detection apparatus for detecting an inclination of 0.5degrees or less with high precision and (3) an optical informationprocessing apparatus for realizing stable information recording to andreproduction from an information memory medium which is significantlycurved.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an optical system of an optical headdevice according to the present invention;

FIG. 2 shows detection areas of a light detector of the optical headdevice shown in FIG. 1, and a configuration of a circuit acting as aninformation reproduction signal generator and a tracking error signalgenerator;

FIG. 3 is a graph illustrating the tracking error signal vs. off-trackamount relationship of the optical head device shown in FIG. 2;

FIG. 4 shows a light detector useable in the optical head device shownin FIG. 1 having different detection areas from the light detector shownin FIG. 2, and a configuration of a circuit acting as an informationreproduction signal generator and a tracking error signal generator;

FIG. 5 shows a light detector usable in the optical head device shown inFIG. 1 having different detection areas from the light detector shown inFIG. 2.

FIG. 6 shows a light detector usable in the optical head device shown inFIG. 1 having different detection areas from the light detector shown inFIG. 2.

FIG. 7 shows a light detector useable in the optical head device shownin FIG. 1 having different detection areas from the light detector shownin FIG. 2, and a configuration of a circuit acting as a tracking errorsignal generator;

FIG. 8 shows a light detector usable in the optical head device shown inFIG. 1 having different areas from the light detector shown in FIG. 2,and a configuration of a circuit acting as a tracking error signal;generator;

FIG. 9 shows a light detector usable in the optical head device shown inFIG. 1 having different detection areas from the light detector shown inFIG. 2, and a configuration of a circuit acting as a tracking errorsignal generator;

FIG. 10A is A schematic view of an optical system, using a lightreducing element, of an optical head device according to the presentinvention;

FIG. 10B is a front view of an objective lens usable in the opticalsystem shown in FIG. 10A;

FIG. 11 in a schematic view of an optical system, using another lightreducing element, of an optical head device according to the presentinvention;

FIG. 12 is a schematic view of an optical system, using still anotherlight reducing element, of an optical head device according to thepresent invention;

FIG. 13 is a schematic view of an optical system using still anotherlight reducing element, of an optical head device according to thepresent invention;

FIG. 14 is a schematic view of an optical system of an optical headdevice according to the present invention.

FIG. 15 shows a relationship among a pattern for dividing a holographicelement, detection areas of the light detector, and the cross section ofthe diffraction light on the light detector in the optical head deviceshown in FIG. 14;

FIG. 16 shows another pattern for dividing a holographic element;

FIG. 17 is a schematic view of an optical system of an optical headdevice according to the present invention;

FIG. 18 shows detection areas a light detector of the optical headdevice shown in FIG. 17, and a configuration of a circuit acting as aninformation reproduction signal generator and a tracking error signal,generator;

FIG. 19A shows a pattern of a groove, tracks and pits of an informationmemory medium;

FIG. 19B shows detection areas of a light detector and a configurationof an information reproduction signal generator for producinginformation stored off track according to the present invention;

FIG. 19C shows different detection areas of a light detector and aconfiguration of an information reproduction signal generator forproducing information stored off track according to the presentinvention:

FIG. 20 shows detection areas of a light detector usable in the opticalhead device shown in FIG. 17, and a configuration of a circuit acting asan information reproduction signal generator and a tracking error signalgenerator;

FIG. 21A shows a pattern of a groove, tracks and pits of an informationmemory medium;

FIG. 21B shows detection areas of a light detector and a configurationof an information reproduction signal generator for producinginformation stored off track according to the present invention;

FIG. 21C shows different detection areas of a light detector and aconfiguration of an information reproduction signal generator forproducing information stored off track according to the presentinvention;

FIG. 22A shows a pattern of a groove, tracks and pits of an informationmemory medium;

FIG. 22B shows detection areas of a light detector and a configurationof an information reproduction signal generator for producinginformation stored off track according to the present invention;

FIG. 23A shows a pattern of a groove, racks and pits of an informationmemory medium;

FIG. 23B shows detection areas of a light detector and a configurationof an information reproduction signal generator for producinginformation stored off track according to the present invention;

FIG. 24 shows a relationship among a pattern for dividing a holographicelement, detection areas of the light detector, and the cross section ofthe diffraction light on the light detector in an optical head deviceaccording to the present invention;

FIG. 25 is a schematic view of an optical system of an optical headdevice according to the present invention;

FIG. 26 shows a relationship among a pattern for dividing a holographicelement, detection areas of the light detector, and the cross section ofthe diffraction light on the light detector in an optic head deviceaccording to the present invention;

FIG. 27 shows another pattern for dividing a holographic element;

FIG. 28 is a schematic view of an inclination detection apparatusaccording to the present invention;

FIG. 29 shows a configuration of a signal processing section of theinclination detection apparatus shown in FIG. 28;

FIG. 30 shows a configuration of an information memory medium usable inthe inclination detection apparatus shown in FIG. 28;

FIG. 31A is a schematic partial view of the track of the informationmemory medium shown in FIG. 30;

FIGS. 31B through 31E show waveforms of a signal which is output fromdifferent elements of the signal processing section shown in FIG. 29;

FIG. 32 is a graph illustrating the relationship between the inclinationdetection signal and the radial inclination of the optical memory mediumobtained in an inclination detection apparatus according to the presentinvention;

FIG. 33 shows another Configuration of a signal processing sectionusable in an inclination detection apparatus according to the presentinvention;

FIG. 34 shows a configuration of an information memory medium usable inan inclination detection apparatus according to the present invention;

FIG. 35 shows still another configuration of a signal processingapparatus in an inclination detection apparatus according to the presentinvention;

FIG. 36 is a schematic view of an optical head device according to thepresent invention;

FIG. 37 shows a polarization filter usable in the optical head deviceshown in FIG. 36;

FIG. 38 shows a configuration of a signal processing section of theoptical head device shown in FIG. 36;

FIG. 39 is a schematic view of an optical head device according to thepresent invention;

FIG. 40 shows a schematic pattern of an off-axis fresnel zone plateformed on a holographic element usable in the optical head device shownin FIG. 39;

FIG. 41 shows the relationship between detection areas of a lightdetector and beams in the optical head device shown in FIG. 39;

FIG. 42 is a schematic view of an optical system of a conventionaloptical head device;

FIG. 43A through 43C each show a pattern of light detection areas of alight detector and the shape of a cross section of light detector by thedetection areas in a conventional optical head device;

FIG. 44 is a schematic view of a conventional inclination detectionapparatus;

FIG. 45 shows a configuration of a conventional information memorymedium:

FIG. 46 shows a configuration of a signal processing section of aconventional inclination detection apparatus;

FIG. 47 is a schematic view of another conventional optical head device;

FIG. 48 shows a configuration of a signal processing section of aconventional optical device; and

FIG. 49 shows a configuration of a conventional information memorymedium used in the optical head device shown in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter the present invention will be described by way of exampleswith reference to the attached drawings. In the following examples,identical reference numerals indicate elements having identicalfunctions, respectively.

EXAMPLE 1

An optical head device according to a first example of the presentinvention will be described with reference to FIGS. 1 through 3.

FIG. 1 is a schematic view of an optical system of the optical headdevice. The optical head device operates in the following manner.

Light emitted by a semiconductor laser 101 (used as a light source) isreflected by a plane-parallel beam splitter 102 and then collimated by acollimator lens 103, which is a part of the optical system. The light iscollected by an objective lens a 104, which is also a part of theoptical system, and then focused on an information layer 108 of anoptical disk 105 employed as an information memory medium. An actuator107 moves the objective lens 104 and a holding device 106 for holdingthe objective lens 104 in accordance with the fluctuation ordecentration of the optical disk 105.

The light is then diffracted and/or reflected by the information layer109 of the optical disk 105 to light 108 a, which its transmitted backthrough the objective lens 104 to be collimated. The collimated light108 a is then converged by the collimator lens 103. The converged light108 a is provided with a non-point aberration when passing through theplane-parallel beam splitter 102. The light after passing through theplane-parallel beam splitter 102 will be referred to as light 108 a. Theconverged light 108 a provided with the non-point aberration is detectedby a light detector 150. The optical system is arranged so that, whenthe focal point F0 of the light from the objective lens 104 is on theinformation layer 108 of the optical disk 105, the detection surface ofthe light detector 150 is at the least circle of confusion of theconverged light 108 a.

The optical disk 105 serving as the information memory medium includes aplurality of grooves. As shown in FIG. 1, the distance between thecenter of one of such grooves and the center of an adjacent groove isindicated by Gp. Information can be stored (1) either on the bottom ofthe groove or between the grooves, or (2) both on the bottom of thegroove and between the grooves. Further in this specification, thenumerical aperture of the objective lens 104 is indicated by NA, and thewavelength of the light emitted by the semiconductor laser 101 isindicated by λ.

In the first example, the optical head device which fulfills theconditions represented by expression (4) will be described.

λ/(NA.Gp)<1   (4)

FIG. 2 shows detection areas 261 through 208 of the light detector 150,an information reproduction signal generator 450, and a tracking errorsignal generator 451.

The light detector 150 is divided into detection areas 201 through 208by division lines 301 through 304. Under the conditions represented byexpression (4), plus first-order diffraction light dad minus first-orderdiffraction light overlap each other at least partially. In FIG. 2, theoverlapping area 200 is indicated by hatching. The maximum value W ofthe width in the radial direction of the overlapping area 200 is:

W=2x(1−λ/(NA×Gp))  (5)

where the aperture of the optical system as shown in FIG. 1 has a radiusof 1.

The division lines 301 through 303 are parallel to the tangent to thegrooves of the optical disk 105 FIG. 1). The division line 304 isperpendicular to the tangent to the grooves of the optical disk 105. Thedirection of the tangent to the grooves of the optical disk 105 is thedirection in which the grooves are optically projected on the lightdetector 150.

In the case where the non-point aberration method is used for detectinga focusing error signal, when a non-point aberration is provided using adirection which is 45 degrees with respect to the grooves of the opticaldisk 105 as the axis, the direction in which the grooves are projectedon the light detector 150 rotates by 90 degrees. Accordingly, even whenthe actual division line is perpendicular to the tangent to the groovesof the optical disk 105, such a division line is represented as being“parallel to the tangent to the grooves” in this specification, as longas such a division line is parallel to the tangent to the groovesprojected on the light detector 150.

The division line 302 passes through the center of the light which isprojected on the light detector 150 through the aperture of theobjective lens 104 (FIG. 1). The division lines 301 and 303 are disposedso as to sandwich the overlapping area 200. Distance a between thedivision lines 301 and 302 and distance a between the division lines 302and 303 is each set to be equal to W/2. By such an arrangement, as shownin FIG. 2, the overlapping area 200 of the plus first-order diffractionlight and the minus first-order diffraction light obtained by theoptical disk 105 is included within the detection areas 202, 203, 206and 207 of the light detector 150. Signals which are obtained, inaccordance with the amount of light detecting by the detection areas 201through 208 are respectively indicated by s1 through s8.

A focusing error signal FE is obtained by expression (6).

 FE=(s1+s2+s5+s6 )−(s3+s4+s7+s8 )  (6)

In FIG. 2, an operation circuit for obtaining the focusing error signalFE is not shown.

Hereinafter, the information reproduction signal generator 450 forgenerating an RF signal, which is an information reproduction signal,will be described. The information reproduction signal generator 450includes adders 401 through 405. The RF signal is obtained based on thesum of the signals obtained from all the detection areas 201 through208. As shown in FIG. 2, the adder 401 outputs the sum of “signals1+signal s8”, the adder 402 outputs the sum of “signal s2+signal s7”,the adder 403 outputs the sum of “signal s3+signal s6”, and the adder404 outputs the SUM Of “signal s4+signal s5”.

The adder 405 receives the outputs from the adders 401 through 404 andoutputs the sum of the four outputs. The output from the adder 405,which is the RF signal, can be represented by expression (7).

RF=s1+s2+s3+s4+s5+s6+s7+s8   (7)

Hereinafter, the tracking error signal generator 451 will be described.As shown in FIG. 2, the tracking error signal generator 451 includes theadders 401 and 404, and a differential operation circuit 406. Thedifferential operation circuit 406 receives the outputs from the adders401 and 404, and outputs the difference between the two outputs. Theoutput from the differential operation circuit 406, i.e., a trackingerror signal TE1, can be represented by expression (8).

TE1 =(s1+s8 )−(s4+s5 )  (8)

The conventional optical head devices have the following drawbacks intracking control when a tilt occurs (i.e., the optical disk 105 tiltswith respect to the radial direction). The radial direction is thedirection perpendicular to the tangent to the grooves of the opticaldisk 105.

When the tilt occurs, the reflected light which is detected is offset inthe direction of the tilt. When a tilt of the disk of θ occurs, thereflected light is offset by 2θ. For example, where the numericalaperture NA is 0.6, the angle θ of tilt is 0.8 degrees, and the apertureis a substantial circle having a radius of 1, sin2θ/NA=0.047. Thereflected light which is detected is offset by 0.047 in the direction ofthe tilt.

Where TE0 is a value of the tracking error signal which is obtained whenthe focal point F0 of the light from the objective lone 104 is at thecenter of the groove, TEmax is this maximum value of the tracking errorsignal which is obtained when the light has crossed the groove, andTEmin is the minimum value of the tracking error signal which isobtained when the light has crossed the groove; the difference between|Temax−TE0| (difference between the absolute values of TEmax and TE0)and |Temin−TE0| (difference between the absolute values of TEmin andTE0) is increased.

FIG. 3 is a graph illustrating the tracking error signal vs. off-trackamount relationship. The off-track amount is the distance of the focalpoint F0 from the center of the track.

The curve having black squares represents the tracking error signalobtained when no aberration is provided. The solid line curve representsthe tracking error signal obtained when a tilt occurs. The solid linecurve has the TEmax of 0.38, the TEmin of −0.42, and the TE0 of −0.20.

|TEmax−TE0 |=0.58, and

|TEmin−TE0 |=0.22.

The difference between |TEmax−TE0| and |TEmin−TE0| is significant.

When tracking control is performed in such a state, the focal point F0of the light from the objective lens 104 a FIG. 1) is off the center ofthe track, and thus information cannot be accurately recorded orreproduced.

In order to position the focal point F0 at the center of the track, anoffset voltage is applied to the tracking error signal, as describedbelow.

In FIG. 3, the curve having white triangles represents the trackingerror signal which is obtained by correcting the off-track amount tozero. An offset voltage is applied so that the center of the trackmatches the zero-cross point of the tracking error signal. At thispoint, the upper amplitude of the tracking error signal, from 0 leveland the lower amplitude thereof from 0 are asymmetrical with each other.

When the upper amplitude end the lower amplitude becomes excessivelyasymmetrical, the tracking control becomes unstable, which preventsaccurate recording and reproduction of information.

As described above, in the conventional optical head devices, when thedifference between |TEmax−TE0| and |TEmin−TE0| becomes excessive, theoff-track amount excessively large. Even if an offset voltage is appliedin order to reduce the off-track amount, the upper amplitude and thelower amplitude of the tracking error signal becomes excessivelyasymmetric, as represented by the excessive degree of asymmetry of theupper and lower amplitudes which is obtained when the off-track amountis corrected to zero.

Comparison of the degrees of asymmetry of the tracking error signal inthe first example and a conventional optical head device will bedescribed.

Where the numerical aperture NA of the objective lens 104 is 0.6, thewavelength λ of the light is 0.66 μm, the distance Gp between thecenters of two adjacent grooves of the optical disk 105 (FIG. 1) is 1.48μm, and the radius of the aperture is 1, the plus first-orderdiffraction light and the minus first-order diffraction light have anoverlapping area having a width W of 0.51.

The case where the thickness of the optical disk 105 is 0.6 mm and atilt of 0.4 degrees occurs in the radial direction will be described.

In the conventional optical head device, the off-track amount is about0.07 μm with respect to the center of the track. Application of anoffset voltage so as to realize the off-track amount of zero, the upperand lower amplitudes of the tracking error signal, is asymmetrical witheach other by 31%.

The degree of asymmetry of the tracking error signal is defined by(A+B)/(A−B) where A is the maximum value of the upper amplitude of thetracking error signal and B is the minimum value of the lower amplitudethereof.

In the optical head device shown in FIG. 2, the off-track amount of thelight with respect to the center of the track can be restricted to assmall as about 0.045 μm when the radial tilt of 0.4 degrees occurs, byobtaining the tracking error signal TE1 only from outside, theoverlapping area of the plus first-order diffraction light and the minusfirst order diffraction light. The degree of asymmetry of the trackingerror signal when the off-track amount is corrected to zero is 19%,which is ⅔ of the degree of asymmetry in the conventional optical headdevice.

In the conventional optical head device, when the object lens 104shifts, the detection spot moves on the light detector 150. When theoff-track amount is corrected to zero, the degree of asymmetry of theupper and lower amplitudes becomes excessive. For example, when theobject lens 104 shifts by 150 μm, the degree of asymmetry is 16% whenthe off-track amount is corrected to zero.

In the optical head device shown in FIG. 2, when the object lens 104shifts by 150 μm, the degree of asymmetry con be restricted as low as 3%when the off-track amount is corrected to zero.

As described above, in the optical head device shown in FIG. 2, even ifthe optical disk 105 tilts, stable tracking control can be performs awhile maintaining the off-track amount small. Thus, the optical headdevice shown in FIG. 2 realizes recording and reproduction ofinformation with a sufficiently low error ratio.

With reference to FIG. 4, another optical head device having anidentical effect with that of the optical head device shown in FIG. 2will be described.

FIG. 4 shows a light detector 151 having detection areas 201, 204, 205,209 and 209, an information, reproduction signal generator 450, and atracking error signal generator 451 of the optical head device shown inFIG. 2. The optical head device shown in FIG. 4 is different from theoptical head device shown FIG. 2 in that the light detector 151 is usedinstead of the light detector 150. The arrangement of the optical systemits identical as that of the optical head device shown in FIG. 2.

The light detector 151 is divided into detection area 201, 204, 205, 208and 209 by division lines 301, 303, 305 and 306. The division lined 301and 303 are disposed so as to sandwich an overlapping area of plugfirst-order diffraction light and minus first-order diffraction light,and the light corresponding to the overlapping area 200 is incident onthe detection area 209 defined by the division lines 301 and 303.

The detection areas 201, 204, 205, 208 and 209 respectively generatesignals s1, s2, s3,; s4 and s5 in accordance, with the amount or lightreceived.

Hereinafter, the information reproduction signal generator 450 will bedescribed. As shown in FIG. 4, the information reproduction signalgenerator 450 includes adders 401, 404 and 405. An RF signal, which isan information reproduction signal, is obtained based on the sum of thesignals obtained from all the detection areas 201, 204, 205, 208 and209. As shown in FIG. 4, the adder 401 outputs the sum of “signals1+signal s4”, and the adder 404 outputs the sum of “signal s2+signals3”. The adder 405 receives the outputs from the adders 401 and 404 andthe signal s5 from the detection area 209, and output the sum of thethree outputs. The output from the adder 405, which is the RF signal,can be represented by expression (9).

RF=s1+s2+s3+s4+s5   (9)

Hereinafter, the tracking error signal generator 451 will be described.As shown in FIG. 4, the tracking error signal generator 451 includes theadders 403, and 404, and a differential operation circuit 406. Thedifferential operation circuit 406 receives the outputs from the adders401 and 404, and outputs the difference between the two outputs. Theoutput from the differential operation circuit 406, i.e., a trackingerror signal TE1, can be represented by expression (10).

TE1 =(s1+s4 )−(s2+s3 )  (10)

In the optical head device shown in FIG. 4, the identical effect as thatof the optical head device shown in FIG. 2 can be obtained with asmaller number of detection areas and head amplifiers.

With reference to FIG. 5, still another optical head device having theidentical effect with that of the optical head device shown in FIG. 2will be described.

FIG. 5 shows a light detector 152 having eight detection areas definedby division lines 306 through 309. The division lines 307 and 309 whichare perpendicular to the tangent to the grooves of the optical disk aredisposed so as to sandwich an overlapping area 200 of plus first-orderdiffraction light and minus first-order diffraction light. Detectionareas 210 through 213 which are outside the division lines 307 and 309and do not include the center of the aperture respectively generatesignals s1, s2, s3, and s4 in accordance with the amount of lightreceived.

The tracking error signal TE1 can be obtained by expression (11).

 TE1 =(s1+s4 )−(s2+s3 )  (11)

In such a structure, the diffraction light can be received except forthe major part of the overlapping area 200 of the plus first-orderdiffraction light and the minus first-order diffraction light.Accordingly, the characteristics of the tracking error signal can beimproved as satisfactorily as the optical head devices shown in FIGS. 2and 4. Thus, the optical head device partially shown in FIG. 5 performsstable tracking control with a relatively small off-track amount, andthus realizes recording and reproduction of information with asufficiently low error ratio.

With reference to FIG. 6, still another optical head device having theidentical effect with that of the optical head device shown in FIG. 2will be described.

FIG. 6 shows a light detector 153 having eight detection areas definedby division lines 310 through 313. The division lines 310 and 312 whichare parallel to the tangent to the grooves of the optical disk aredisposed near the periphery of the cross section of the light 108 a(FIG. 1) on the light detector 153. Detection, areas 214 through 217which are outside the division lines 310 and 112 and do not include thecenter of the circle A respectively generate signals s1, s2, s3, and s4in accordance with the amount of light received.

The tracking error signal TE1 can also be obtained by expression (11).

In such a structure, the diffraction light can be received from theareas sufficiently far from the overlapping area 200 of the plusfirst-order diffraction light and the minus first-order diffractionlight. Accordingly, the characteristics of the tracking error signal canbe improved as satisfactorily as the optical head devices shown in FIGS.2 and 4. Thus, the optical head device partially shown in FIG. 6performs stable tracking control with a relatively small off-trackamount, and thus realizes recording and reproduction of information witha sufficiently low error ratio.

In FIGS. 2, 4, 5 and 6, the division lines are straight. Since theoverlapping area 200 of the plus first-order diffraction light and theminus first-order diffraction light is enclosed by a curve, the divisionlines may be curved lines in accordance with the overlapping area 200.

EXAMPLE 2

In a second example according to the present invention, a tracking errorsignal is corrected based on a signal obtained from the overlapping areaof the plus first-order diffraction light and the minus first-orderdiffraction light which are obtained by the grooves of the optical disk105 (FIG. 1). In the second example also, the conditions represented byexpression (4) described above are fulfilled.

The structure and operation of an optimal head device in the secondexample are substantially the same as those of the optical system shownin FIG. 1, and thus detailed description thereof will be omitted.

FIG. 7 shows a light detector 150 including detection areas 203 through208, and a tracking error signal generator 451 of an optical head deviceaccording to the second example of the present invention.

The light detector 150 is divided into the eight detection areas 201through 208 by division lines 301 through 304. The division lines aredisposed in the came manner as shown in FIG. 2. The detection areas 201through 208 respectively generate signals s1 through s8 in accordancewith the amount of light received. The method for generating thefocusing error signal FE and the information reproduction signal artsthe same as described in the first example with reference to FIG. 2 andwill not be described in the second example.

As shown in FIG. 7, the tracking error signal generator 451 includesadders 401 through 404, 407 and 408, and a differential operationcircuit 406. As shown in FIG. 7, the adder 401 outputs the sum of“signal s1+signal s8”, the adder 402 outputs the sum of “signals2+signal s7”, the adder 403 outputs the sum of “Signal s3+signal s6”,and the adder 404 outputs the sum of “signal s4+signal s5”. The adder407 receives the outputs from the adders 401 and 403 and outputs the sumof the two outputs. The adder 408 receives the outputs from the adders402 and 404 and outputs the sum of the two outputs. The differentialoperation circuit 406 receives the outputs from the adders 407 and 408,and outputs the difference between the two outputs.

The output from the differential operation circuit 406, i.e., a trackingerror signal, can be represented by expression (12).

TE1 =(s1+s3+s6+s8 )−(s2+s4+s5+s7 )  (12)

The method of processing a signal generated by the detection areas whichreceive the light corresponding to the overlapping area 200 of the plusfirst-order diffraction light and the minus first-order diffractionlight, which is carried out by the structure of FIG. 7, is differentfrom the usual push-pull method. In the structure of FIG. 7, thetracking error signal is corrected by inverting at least one of thesignals generated by the detection areas which receive the lightcorresponding to the overlapping area 200 and then adding together theinverted signal and the other signals.

In the optical head device shown in FIG. 7, when a tilt of 0.4 degreesoccurs in the radial direction the degree of asymmetry of the upper andlower amplitudes of the signal with respect to zero is 14% even when theoff-track amount is corrected to zero. This degree of asymmetry is about½ of the degree obtained in the conventional optical head device. Thestructure and method for correcting the tracking error signal describedwith reference to FIG. 7 is specifically advantageous for restrictingthe asymmetry against a radial tilt.

As described above, the optical head device shown in FIG. 7 performsstable tracking control with a relatively small off-track amount even ifa tilt of the optical disk 105 (FIG. 1) occurs, and thus realizesrecording and reproduction of information with a sufficiently low errorratio.

With reference to FIG. 8, another optical head device having theidentical effect with that of the optical head device shown in FIG. 7will be described.

FIG. 8 shows a light detector 153 including 12 detection areas 218through 229 defined by division lines 315 through 320, and a trackingerror signal generator 451.

The division lines 315 and 319 are parallel to the tangent to thegrooves of the optical disk 105 (FIG. 1) and disposed so as to interposeoverlapping area 200 of the plus first-order diffraction light and theminus first-order diffraction light obtained by the grooves of theoptical disk 105. The distance between the division lines 315 and 319 isapproximately the same as W in expression (5) described above withreference to FIG. 2. The division lines 316 through 318 are disposedequally spaced between the division lines 315 and 319. The detectionareas 218 through 229 respectively generate signals s1 through s12 inaccordance with the amount of light received.

As shown in FIG. 8, the tracking error signal generator 451 includesadders 401 through 404 and 409 through 412, and a differential operationcircuit 406.

As shown in FIG. 8, the adder 401 outputs the sum of “signal s1+signals12”, the adder 402 outputs the sum of “signal s2+signal s11”, the adder403 outputs the sum of “signal s3+signal s10”, the adder 404 outputs thesum or “signal s4+signal s9”, the adder 409 outputs the sum of “signals5+signal s8”, and the adder 410 outputs the sum of “signal s6+signals7”. The adder 411 receives the outputs from the adders 401, 403, and409 and outputs the sum of the three outputs. The adder 412 receives theoutputs from the adders 402, 404 and 410 and outputs the bum of thethree outputs. The differential operation circuit 406 receives theoutputs from the adders 411 end 412, and outputs the difference betweenthe two outputs.

The output from the differential operation circuit 406, i.e. a trackingerror signal, can be represented by expression (13).

TE1 =(s1+s3+s5+s8+s10+s12 )−(s2+s4+s6+s7+s9+s11 )  (13)

In the optical head device shown in FIG. 8, even when a tilt of 0.4degrees occurs in the radial direction, the off-track amount is merely0.042 μm.

When the off-track amount is corrected to zero, the degree of asymmetryof the upper and lower amplitudes of the tracking error signal withrespect to zero is 18%, whereas the degree of asymmetry in theconventional optical head device is 31%. As can be understood from this,the optical head device shown in FIG. 8 provides a smaller degree ofasymmetry than in the conventional optical head device even when theoff-track amount is corrected to zero.

When the objective lens shifts by 150 μm, the degree of asymmetry in theoptical head device shown in FIG. 8 is 6% when the off-track amount iscorrected to zero whereas the degree of asymmetry in the conventionaloptical head device is 16%. As is apparent from this, the optical headdevice shown in FIG. 8 reduces the asymmetry of the tracking errorsignal to about half compared to the conventional optical head device.

As described above, the optical hood device shown in FIG. 8 performsstable tracking control with a relatively small off-track amount even ifa tilt of the optical disk 105 occurs, and thus realizes recording andreproduction of information with a sufficiently low error ratio.

In the second example, the overlapping area 200 of the plus first orderdiffraction light and the minus first-order diffraction light is dividedinto four in the direction perpendicular to the tangent to the groovesof the optical disk 106. The overlapping area 200 may be divided into agreater number of areas, in which case, the same affect can be obtained.

The division line 320 which is perpendicular to the tangent to thegrooves of the optical disk is provided in consideration of thenon-point aberration method used for focusing the light. In the casewhere the division line 320 is not provided, the adder 401 through 404,409 and 410 are not necessary. In such a case, the same effect can beobtained.

EXAMPLE 3

In a third example according to the present invention, a tracking errorsignal is corrected by generating a correction signal corresponding todisturbance from a signal obtained from the overlapping area of the plusfirst-order diffraction light and the minus first-order diffractionlight which are obtained by the grooves of the optical disk 105 (FIG.1). In the third example also, the conditions represented by expression(4) described above are fulfilled.

The structure and operation of an optical head device in the thirdexample are substantially the same as those of the optical system shownin FIG. 1, and thus detailed description thereof will be omitted.

FIG. 9 shows a light detector 150 including detection areas 201 through208, and a tracking error signal generator 451 of an optical head deviceaccording to the third example of the present invention.

The light detector 150 is divided into the eight detection areas 201through 208 by division lines 301 through 304. The division lines aredisposed in the same manner as shown in FIG. 2. The detection areas 201through 208 respectively generate signals s1 through s8 in accordancewith the amount of light received.

The method for generating the focusing error signal FE and theinformation reproduction signal are the same as described in the firstexample with reference to FIG. 2 and will not be described in the thirdexample.

As shown in FIG. 9, the tracking error signal generator 451 includesadders 401 through 404, variable gain amplification circuits 413 and414, and differential operation circuits 406, 415 and 416.

The variable gain amplification circuit 414 receives an output from theadder 402 and outputs a signal obtained by multiplying the output fromthe adder 402 by α1, (α1 being a prescribed value). The variable gainamplification circuit 413 receives an output from the adder 403 andoutputs a signal obtained by multiplying the output from the adder 403by α2, (α2 being a prescribed value).

The differential operation circuit 406 receives outputs from the adders401 and 404 and outputs a signal obtained by subtracting the output ofthe adder 404 from the output of the adder 401.

The differential operation circuit 415 receives the outputs from thevariable gain amplification circuits 413 and 414, and outputs theirdifference.

The differential operation circuit 416 receives the outputs from theirdifferential operation circuits 406 and 415, and outputs theirdifference.

The output from the differential operation circuit 416 (i.e., a trackingerror signal) can be represented by expression (14).

TE1=(s1+s8)−(s4+s5)−{α1·(s2+s7)−α2·(s3+s6)}  (14)

The gain α1 of the variable gain amplification circuit 414 and gain α2of the variable gain amplification circuit 413 are determined inconsideration of the amount and polarity of the radial tilt and theamount and polarity of the shift of the objective lens. By changing thegains α1 and α2, the difference between |TEmax−TE0| and |TEmin−TE0| canbe reduced. In other words, the correction amount can be reduced byadjusting the gains. The optical head device shown in FIG. 9 accordingto the third example is more advantageous over the optical head devicesof the second example with respect to improving the amount ofcorrection.

As described above, the optical head device shown in FIG. 9 providesstable tracking control with relatively small off-track error even if atilt of the optical disk 105 (FIG. 1) occurs, and thus realizesrecording and reproduction of information with a substantially low errorratio.

In the third example, the overlapping area 200 of the plus first-orderdiffraction light and the minus first-order diffraction light is dividedinto two in the direction perpendicular to the tangent to the grooves ofthe optical disk 105. The overlapping area 200 may be divided into agreater number of areas, in which case, the same effect can be obtained.

EXAMPLE 4

In a fourth example according to the present invention, characteristicsof a tracking error signal are improved by reducing the opticaltransmission of an area including an overlapping area of the plusfirst-order diffraction light and the minus first-order diffractionlight which are obtained by the grooves of the optical disk 105 (FIG.10A). In the fourth example, the conditions represented by expression(4) described above also are fulfilled.

FIG. 10A is a schematic view of an optical system of an optical headdevice according to the fourth example of the present invention. Theoptical system in FIG. 10A is identical with that of the optical systemshown in FIG. 1 except with respect to the objective lens 109.Accordingly, a detailed description of the optical system except for theobjective lens 109 will be omitted.

FIG. 10B is a front view of the objective lens 109 which is included inthe collection optical system of the optical head device shown in FIG.10A. A holographic element is provided on the objective lens 109 in ahatched area 110 as a light reducing element. The light diffracted bythe holographic element becomes unnecessary light, which does notcontribute to the reproduction of the signal. Only zero-the order lightwhich is not diffracted by the holographic element is effectivelycollected on the information layer 108 of the optical disk 105.

Detection areas 251 through 254 of a light detector 150 can be arrangedas shown in FIG. 43A. In the case where the detection areas 251 through254 are arranged as shown in FIG. 43A, a tracking error signal TE1, afocusing error signal FE, and an information reproduction signal RF areobtained by the expressions (1), (2) and (3), respectively.

Comparison of the degrees of asymmetry of the tracking error signal inthe fourth example of a conventional optical head device will bedescribed.

The comparison is performed by correcting the off-track amount to zerounder the same conditions as those described regarding the optical headdevice shown in FIG. 2, in the case where the transmittance T of thearea 110 is 55%, the ratio of the radius of the area 110 with respect tothe radius of the objective lens 109 is 0.72, and the angle of theradial tilt is 0.4 degrees. The degree of asymmetry of the upper andlower amplitudes of the tracking error signal is restricted to 17% inthe optical head device shown in FIG. 10A, whereas the degree ofasymmetry is 31% in the conventional optical head device. The degree ofasymmetry obtained in the optical head device shown in FIG. 10A is about½ of the degree obtained in the conventional optical head device.

As described above, in the optical head device shown in FIG. 10A, evenif the optical disk 105 tilts, stable tracking control can be performedwhile maintaining the off-track amount small. Thus, the optical headdevice shown in FIG. 10A realizes recording and reproduction ofinformation with a sufficiently low error ratio.

In this example, another light spot may be formed by light diffracted bythe holographic element provided on the objective lens 109. In such acase, zero-the order diffraction light can be used to form a spot for aDVD having a thickness of 0.6 mm, and first-order diffraction light canbe used to form a spot for a CD having a thickness of 1.2 mm thick. Byforming a different light spot by light diffracted on the holographicelement, the function of a bifocal lens can be provided.

In this example, the holographic element is provided as a light reducingelement. The same effect can be obtained by providing the objective lenswith a reflection film having a appropriate transmittance or providingthe objective lens with a light absorption film.

Alternatively, as shown in FIG. 11, a holographic element or filter 111including a light reducing film may be held so as to be moved withrespect to the objective lens 104, in lieu of directly attaching any ofthe above-mentioned light reducing elements on the objective lens. Insuch a case, the same effect can be obtained.

In FIG. 11, the filter 111 as a light reducing element covers a surfaceof the objective lens 104 which is farther from the optical disk 105,but may cover a surface of the object lens 104 which is closer to theoptical disk 105. Similarly, the light reducing element in FIG. 10A cancover a surface of the objective lens 10 which is closer to the opticaldisk 105.

In the fourth example, the light reducing element is provided integrallywith the collection optical system, but may be provided in the fixedoptical system separately from the collection optical system. FIG. 12shows an exemplary optical head device including a light reducingelement on a light path from the light source to the optical disk. Asshown in FIG. 12, the light emitted by the semiconductor laser 101 as alight source passes through a filter 111 as a light reducing element. Bythe filter 111, the light corresponding to a central area of the filter111 is reduced. The remaining light is reflected by the plane-parallelbeam splitter 102. In such a structure, the zero the order diffractionlight in the overlapping area of the plus first-order diffraction lightand the minus first order diffraction light which are obtained from thegrooves of the optical disk 105 is reduced. Thus, the influence of theoverlapping area can be reduced.

FIG. 13 shows an exemplary optical head device including a lightreducing element in an optical system from the optical disk to the lightdetector 150. In FIG. 13, a light reducing element 113 is provided on abottom surface of a plane-parallel beam splitter 112. In such anarrangement also, the influence of the over-lapping area of the plusfirst-order diffraction light and the minus first-order diffractionlight can be reduced, and the same effect can be obtained.

In the first through fourth examples, the non-point aberration method isused for obtaining a focusing error signal. The present invention is notlimited to such a method. The non-point aberration method can be used incombination with other methods such as the Foucault method or the spotsize method.

EXAMPLE 5

In a fifth example according to the present invention, a holographicelement or a stepped prism is used as a light division element. Afocusing error signal is obtained by the spot method. In the fifthexample also, the conditions represented by expression (4) describedabove are fulfilled.

An optical head device according to the fifth example of the presentinvention will be described with reference to FIG. 14. FIG. 14 is aschematic view of an optical system of the optical head device. Theoptical head device shown in FIG. 14 operates in the following manner.

Light emitted by a semiconductor laser 101 (used as a light source) iscollimated by a collimator lens 103 and then reflected by a beamsplitter 115. Then, the light is converged by an objective lens 104,which is a part of the optical system, and then collected on aninformation layer 108 of an optical disk 105 employed as an informationmemory medium. The light is then reflected by the information layer 108of the optical disk 105 as light 108 a, which is transmitted backthrough the objective lens 104 to be collimated. The collimated light108 a is then transmitted through the beam splitter 115 and converged bya detection lens 116. The converged light 108 a is diffracted by aholographic element 117 provided as a light division element. Minusfirst-order diffraction light 108 b and plus first-order diffractionlight 108 c obtained by the diffraction performed by the holographicelement 117 are received by a light detector 154.

FIG. 15 shows a pattern for dividing the holographic element 117, adetection area of the light detector 154, and cross sections of theminus first-order diffraction light 108 b and the plus first-orderdiffraction light 108 c on the light detector 154.

The holographic element 117 is divided into a plurality of strip-shapedareas A, a, B, b, C, c, D and d. These letters indicating thestrip-shaped areas also indicate the corresponding cross sections of theminus first-order diffraction light 108 b and the plus first-orderdiffraction light 108 c. The minus first-order diffraction light 108 bobtained from the areas represented by capital letters A, B, C and D iscollected farther from the minus first-order diffraction light 108 bobtained from the areas represented by lower case letters a, b, c and dwith respect to the detection lens 116.

The optical system and the holographic element 117 are designed so that,when the focal point F0 of the light from the objective lens 104 matchesthe information layer 108 of the optical disk 105, the cross section ofthe diffraction light obtained from the areas represented by the capitalletters A, B, C and D has an equal size to that of the cross section ofthe diffraction light obtained from the areas represented by the lowercase letters a, b, c and d.

Detection areas 230, 231 and 232 of the light detector 154 generatessignals f1, f2 and f3 in accordance with the amount of light received.

A focus error signal FE is obtained by expression (15).

FE=F1+f3−f2  (15)

When the information layer 108 is distanced from the objective lens 104to be beyond the focal point F0 of the light from the objective lens104, the cross sections A, B, C and D of the minus first-orderdiffraction light 108 b are decreased, and the cross sections a, b, cand d of the minus first-order diffraction light 108 b are increased.Accordingly, the amplitudes of signals f1 and f3 reduce and theamplitude of signal f2 increases. The amplitude of the focusing errorsignal FE reduces.

When the information layer 108 approaches the objective lens 104 to bebetween the objective lens 104 and the focal point F0 of the light fromthe objective lens 104, the cross sections A, B, C and D of the minusfirst-order diffraction light 108 b are increased, and the crosssections a, b, c and d of the minus first order diffraction light 108 bare decreased. Accordingly, the amplitudes of signals f1 and f3 increaseand the amplitude of signal f2 reduces. The amplitude of the focusingerror signal FE increases.

In such a system, focusing control for maintaining the focal point F0 onthe information layer 108 is realized.

Detection areas 233 through 236 of the light detector 154 generatesignals t1 through t4 in accordance with the mount of light received. Anoperation circuit (not shown) generates a tracking error signal TE1.

The tracking error signal TE1 is obtained by expression (16).

TE1=(t1+t4)−(t2+t3)  (16)

This substantially provides the difference in light amount between thehatched areas and the blank areas in FIG. 15.

By the differential phase method, a tracking error signal TE2 isobtained by comparing the phase of (signal t1+signal t3) and the phaseof (signal t2+signal t4).

An RF signal for reproducing the information is obtained as RFf byexpression (17), as RFt by expression (18), or as the sum of RFf andRFt.

RFf=f1+f2+f3  (17)

RFt=t1+t2+t3+t4  (18)

A feature of the optical head device shown in FIG. 15 is that aplurality of areas sandwiching the division line (which runs through thecenter of the aperture and is parallel to the grooves of the opticaldisk) are exchanged with each other diagonally. In FIG. 15, the total ofeight areas, namely, areas A and D, areas B and C, areas a and d, andareas b and c are exchanged with each other. Thus, the same effect asthat of the second example can be obtained. In more detail, when a tiltof 0.4 degrees occurs in a radial direction, the degree of asymmetry ofthe upper and lower amplitudes of the tracking error signal is 14% whenthe off-track amount is corrected to zero. The degree of 14% is about ½of the degree of asymmetry obtained in the conventional optical headdevice. The optical head device shown in FIG. 15 is specificallyadvantageous for restricting a radial tilt.

As described above, in the optical head device shown in FIG. 15, even ifthe optical disk 105 (FIG. 14) tilts, stable tracking control can beperformed while maintaining the off-track amount small. Thus, theoptical head device shown in FIG. 15 realizes recording and reproductionof information with a substantially low error ratio.

FIG. 16 shows another pattern for dividing a holographic element 118. InFIG. 16, only a part of the areas sandwiching the division line (whichruns through the center of the aperture and is parallel to the groovesof the optical disk) are exchanged with each other diagonally. In thiscase, the same effect as that described in the second example withreference to FIG. 8 can be obtained. In the case where the angle of theradial tilt is 0.4 degrees, the off-track amount is 0.042 μm. When theoff-track amount is corrected to zero, the degree of asymmetry of theupper and lower amplitudes of the tracking error signal is 18%. In theconventional optical head device, in the case where the angle of theradial tilt is 0.4 degrees, the degree of asymmetry is 31% when theoff-track amount is corrected to zero. The optical head device shown inFIG. 16 using the holographic element 118 significantly reduces thedegree of asymmetry of the tracking error signal compared to theconventional optical head device.

In the optical head device shown in FIG. 16 using the holographicelement 118, in the case where the objective lens shifts by 150 μm, thedegree of asymmetry of the upper and lower amplitudes of the trackingerror signal is 8% when the off-track amount is corrected to zero. Inthe conventional optical head device, in the case where the objectivelens shifts by 150 μm, the degree of asymmetry is 16% when the off-trackamount is corrected to zero. The optical head device shown in FIG. 16using the holographic element 118 significantly reduces the degree ofasymmetry of the tracking error signal compared to conventional opticalhead devices.

As described above, in the optical head device shown in FIG. 16, even ifthe optical disk 105 (FIG. 14) tilts stable tracking control can beperformed while maintaining the off-track error small. Thus, the opticalhead device shown in FIG. 16 realizes recording and reproduction ofinformation with a substantially low error ratio.

As can be understood from the above description, according to theoptical head device in the fifth example, a significant effect can beachieved by using a holographic element as a light division element,without increasing the number of detection areas of the light detectorcompared to conventional devices. Such a structure does not require thenumber of head amplifiers for generating an RF signal to be increased,and thus the circuit can be simplified.

In the fifth example, the aperture is divided by a division line whichis perpendicular to the tangent to the tracks. Areas a through d andareas A through D are arranged to sandwich the division line. The areasa through d form a spherical wave having a light collection point on theside closer to the collimator lens 103; and areas A through D form aspherical wave having a light collection point on the side farther fromthe collimator lens 103. In addition, these two types of areas arearranged alternately in an area outside the aperture which is notusually irradiated by light.

In the case where the division line perpendicular to the tracks is notprovided and strip-like areas extending parallel to the tracks from oneend to the other end of the aperture are provided, when the holographicelement 117 is offset with respect to the center of the aperture of theobjective lens 104, the balance between the amount of light incident onthe areas a through d of the light detector 154 and the amount of lightincident on the areas A through D is spoiled. When the light collectionpoint F0 of the light from the objective lens 104 is off the informationlayer 108 of the optical disk 105 to cause defocus, stripes of brightareas and dark areas which are parallel to the tracks are formed in anarea where the ±first-order diffraction light formed by the optical disk105 and the zero-th order light overlap. Again, the balance between theamount of light incident on the areas a through d of the light detector154 and the amount of light incident on the areas A through D is spoiledby the manner in which the stripes and the areas of holographic element117 overlap.

On calculation, in a conventional device where there is no division lineperpendicular to the tangent to the tracks, under the conditions wherethe numerical aperture NA of the objective lens is 0.5, the wavelength λof light is 0.795 μm, the diameter of the objective lens is 4 mm, thewidth of each area of the holographic element is 0.2 mm, and theobjective lens is off with respect to the center of the polarizationanisotropic holographic element by 100 μm, the focus gain changes by 3.4dB by the defocus of =2 μm.

In the case of the arrangement according to the fifth example, the focusgain changes only by 1.2 dB under the same conditions on calculation.Such a change is about ⅓ of the conventional device, indicating asignificant reduction. Thus, the focus control is stabilized againstdisturbance. Accordingly, the error ratio in information reproduction isreduced.

In the case where the above-described two types of areas are providedoutside the aperture without any division line perpendicular to thetracks, the focus gain changes only by 1.8 dB under the same conditionson calculation. Such a change is about ½ of the conventional device.Accordingly, the focus control is stabilized, and thus the error ratioin information reproduction is reduced.

The pattern for dividing the holographic element can be set more freelythan the pattern for dividing the light detector. Accordingly, asuitable pattern for a particular purpose can be easily realized.

In the fifth example, only a holographic element is used. Alternatively,a polarizing anisotropic holographic element can be used in combinationwith a ¼-wave plate. In this case, the light utilization factor can beimproved without spoiling the above-described effect of the presentinvention.

In the fifth example, a light division element for performing both thefocusing control and the tracking control is provided. Since thefocusing control and the tracking control are independent from eachother, a light division element for performing either one of them may beprovided. In such a case, the effect in accordance with such a structurecan be obtained.

In lieu of a holographic element, a stepped prism may be used as thelight division element. FIG. 17 is a schematic view of an optical systemincluding a stepped prism 119 in place of the holographic element 117shown in FIG. 14. The light divided by the stepped prism 119 is receivedby the light detector 154. In this case also, the same effect as in thestructure using a holographic element can be obtained.

EXAMPLE 6

In a sixth example according to the present invention, a structure andmethod for reproducing information which is recorded at a position offthe grooves of the optical disk used as an information memory mediumwill be described.

An optical system of the optical head device used in the sixth examplehas substantially the same structure as and operates in the same manneras the optical system shown in FIG. 1, and thus will not described indetail.

FIG. 18 shows detection areas of the light detector 150, an informationreproduction signal generator 450 and a tracking error signal generator451 of the optical head device.

The light detector 150 is divided into eight detection areas 201 through208 by division lines 301 through 304. The division lines 301 through304 are disposed in the same manner as in FIG. 2. The detection areas201 through 208 generate signals s1 through s8 in accordance with theamount of light received.

As shown in FIG. 18, the information reproduction signal generator 450includes adders 401 through 404 and 405, 417 and 419, and differentialoperation circuits 418 and 420. The tracking error signal generator 451includes the adders 401 through 404, 421 and 422, and a differentialoperation circuit 423. The optical head device shown in FIG. 18 furtherincludes an address detection circuit 424 and a control circuit 425.

With reference to FIGS. 19A, 19B and 19C, the operation for reading aseries of pits recorded off the grooves in the optical disk will bedescribed.

FIG. 19A shows a part of the optical disk having a groove 501 and aseries of pits 502 on the information layer 108 (FIG. 1).

A track 507 runs along the center of the grooves 501 in zones 503 and506. In a first address zone 504, a center line 508 of the series ofpits is off the track 507 in one direction by a prescribed distance. Ina second address zone 505, a center line 509 of the series of pits isoff the track 507 in the opposite direction by a prescribed distance.

When the focal point F0 of the light from the objective lens 104(FIG. 1) is in the zone 503 or 506, the focal point F0 is on the track507, for example, on a point 510 in the case where tracking control isperformed. While the optical disk 105 (FIG. 1) rotates and thus moveswith respect to the optical head device, the focal point F0 moves fromthe point 510 to a point 511, a point 512, and a point 513 on anextension of the track 502. While the focal point F0 is in the firstaddress zone 504 and the second address zone 505, the tracking errorsignal is on hold.

A focusing error signal FE is obtained as in the first example based onexpression (6).

FE=(s1+s2+s5+s6)−(s3+s4+s7+s8)  (6)

The tracking error signal THE is generated by the tracking error signalgenerator 451 in the following manner.

The adder 421 receives signal t1 from the adder 401 and signal t2 fromthe adder 402, and outputs the sum of signals t1 and t2. The adder 422receives signal t3 from the adder 403 and signal t4 from the adder 404,and outputs the sum of signals t3 and t4. The differential operationcircuit 423 receives the signals from the adders 421 and 422, andoutputs the difference between the two signals. The output from thedifferential operation circuit 423, (i.e., the tracking signal THE) isobtained based on expression (19).

THE=(t1+t2)−(t3+t4)  (19)

The tracking control is performed by the tracking error signal THE.

The information reproduction signal generator 450 generates aninformation reproduction signal RF in the following manner.

The adder 401 receives signals s1 and s8, and outputs signal t1, whichis the sum of signals s1 and s8. The adder 402 receives signals s2 ands7, and outputs signal t2, which is the sum of signals s2 and s7. Theadder 403 receives signals s3 and s6, and outputs signal t3, which isthe sum of signals s3 and s6. The adder 404 receives signals s4 and s5,and outputs signal t4, which is the sum of signals s4 and s5. The adder405 receives signals t1 through t4 from the adders 401 through 404, andoutputs the sum of the four signals. The output from the adder 405,which is an information reproduction signal RF, is obtained byexpression (20).

RF=t1+t2+t3+t4  (20)

The adder 417 receives signals t2, t3 and t4 from the adders 402, 403and 404, and outputs the sum of the three signals. The differentialoperation circuit 418 receives the signals from the adders 401 and 417,and outputs the difference between the two signals. The output from thedifferential operation circuit 418, (i.e., signal RFa1) is obtained byexpression (21).

RFa1=t1−(t2+t3+t4)  (21)

The adder 419 receives signals t1, t2 and t3 from the adders 401, 402and 403, and outputs the sum of the three signals. The differentialoperation circuit 420 receives the signals from the adders 419 and 404,and outputs the difference between the two signals. The output from thedifferential operation circuit 420, (i.e., signal RFa2) is obtained byexpression (22).

Rfa2=(t1+t2+t3)−t4  (22)

The address detection circuit 424 receives the tracking error signal THEfrom the differential operation circuit 423. Then, the address detectioncircuit 424 determines whether the focal point F0 of the light from theobjective lens is in the zone 503 or 506 having the groove 501, in thefirst address zone 504 or the second address zone 505, and outputs anidentification signal indicating the determination result.

The control circuit 425 receives the identification signal from theaddress detection circuit 424, and controls switches 426 and 427 basedon the identification signal.

The switch 426 receives the signals from the differential controlcircuits 418 and 420, and outputs one of the two signals.

The switch 427 receives the signals from the adder 405 and the switch426, and outputs one of the two signals. The signal output from theswitch 427 is used for reproducing the information or address stored inthe optical disk 105 (FIG. 1).

The control circuit 425 controls the switches 426 and 427 so that, whenthe focal point F0 of the light from the objective lens is in the zone503 or 506 having the groove 501, the switch 427 outputs the signal RFwhich is input from the adder 405. The control circuit 425 also controlsthe switches 426 and 427 so that, when the focal point F0 is in thefirst address zone 504, the switch 427 outputs the signal RFa1 which isinput from the differential operation circuit 418; and so that, when thefocal point F0 is in the second address zone 505, the switch 427 outputsthe signal Rfa2 which is input from the differential operation circuit420.

Through the above-described control of the control circuit 425,information stored in the optical disk is reproduced in the followingmanner.

When the focal point F0 is at the point 511 in the first address zone504, information stored in the series of pits corresponding to the track507 is reproduced based on signal RFa1, which indicates the differencebetween the signals obtained from the two areas (lined area and hatchedarea) sandwiching the division line 301.

When the focal point F0 is at the point 512 in the second address zone505, information stored in the series of pits corresponding to the track507 is reproduced based on signal RFa2, which indicates the differencebetween the signals obtained from the two areas (lines area and hatchedarea) sandwiching the division line 303.

Hereinafter, comparison between the optical head device shown in FIG. 18and the conventional optical head device regarding the jitter will bedescribed. In the following comparison, the numerical aperture NA of theobjective lens is 0.6, the wavelength λ of the light is 0.660 μm, andthe space Gp between two adjacent guiding grooves is 1.48 μm. The centerline 508 of the series of pits 502 in the first address zone 504 and thecenter line 509 of the series of pits 502 in the second address zone 505are each off the center line of the track 507 by 0.37 μm.

In the conventional optical head device, in which the information isreproduced based on a signal indicating the difference among the signalsobtained from each of the areas in the light detector defined by acentral division line of the aperture, the jitter obtained bycalculation is 6.4%.

In the optical head device shown in FIG. 18, the information can bereproduced in the following manner. The division line 302 divides theaperture equally into two. The aperture has a radius of 1. The distancebetween the division lines 301 and 302 is represented by d, and thedistance between the division lines 303 and 302 is also represented byd, where d=0.23. The aperture is divided into two detection areas by thedivision line 301 and 303, and the information is reproduced based on asignal indicating the difference between the signals obtained from thetwo detection areas, namely, signals RFa1 or RFa2. In such a system, thejitter obtained by calculation is 1.8%. Thus, the optical head deviceshown in FIG. 18 can improve the jitter by 4% or more compared to theconventional optical head device.

In the case where d is 0.1 or more and 0.3 or less, the jitter is 3% orless in the optical head device shown in FIG. 18. In this case, such alevel of jitter is half or less of the jitter obtained in theconventional optical head device.

As described above, in the optical head device shown in FIG. 18,information stored in the form of a series of pits which are positionedoff the track can be reproduced with a sufficiently low level of jitter.Accordingly, margin against disturbance and the like is increased, andthus information such as addresses can be recorded to and reproducedfrom the optical disk in the form of pits with satisfactory stability.

With reference to FIG. 20, an optical head device having a differentlight detector 151 from the detector shown in FIG. 18 will be described.

FIG. 20 shows detection areas 201, 204, 205, 208 and 209 of the lightdetector 151, an information reproduction signal generator 450 and atracking error signal generator 451 of the optical head device.

As shown in FIG. 20, the information reproduction signal generator 450includes adders 401, 404, 405, 417 and 419 and differential operationcircuits 418 and 420. The tracking error signal generator 451 includesthe adders 401 and 404, and a differential operation circuit 423. Theoptical head device shown in FIG. 20 further includes an addressdetection circuit 424 and a control circuit 425.

The adder 405 generates a signal corresponding to the total amount oflight received by the detection areas 201, 204, 205, 208 and 209 of thelight detector 151. The differential operation circuit 418 generates adifferential signal corresponding to {signal t1−(signal t2+signal t3)}.The differential operation circuit 420 generates a differential signalcorresponding to {(signal t1+signal t2)−signal t3)}. Signal t1 isgenerated by the adder 401, signal t2 is the same as signal s5 which isoutput from the detection area 209, and signal t3 is generated by theadder 404.

The tracking error signal is a differential signal indicating thedifference between signals t1 and t3. Signal t1 is the sum of thesignals output from the detection areas 201 and 208 which are disposedleft to the division line 301. Signal t3 is the sum of the signalsoutput from the detection areas 204 and 205 which are disposed right tothe division line 303.

Due to the above-described structure, the optical head device shown inFIG. 20 can perform recording and reproduction of information such asaddresses to and from the optical disk with substantial stability.

With reference to FIGS 21A, 21B and 21C, an optical head device having adifferent information reproduction signal generator from the generatorshown in FIG. 18 will be described.

FIG. 21A shows a part of the optical disk having a groove 501 ad aseries of pits 502 on the information layer 108 (FIG. 1).

FIGS. 21B and 21C schematically show a light detector 350 and aninformation reproduction signal generator 450.

FIG. 21B shows a circuit configuration for reproducing informationstored in a first address zone 504 of the optical disk in FIG. 21A. FIG.21C shows a circuit configuration for reproducing information stored ina second address zone 505 of the optical disk in FIG. 21A. Theinformation reproduction signal generator 450 includes an adder 430 anda differential operation circuit 431 shown in FIG. 21B and also an adder432 and a differential operation circuit 433 shown in FIG. 21C.

The differential operation circuit 431 generates a reproduction signalRFa3 based on expression (23), and the differential operation circuit433 generates a reproduction signal RFa4 based on expression (24).

RFa3=t1−(t3+t4)  (23)

RFa4=(t1+t2)−t4  (24)

The reproduction signal RFa3 is used for reproducing the informationstored in the first address zone 504, and the reproduction signal RFa4is used for reproducing the information stored in the second addresszone 505. The information reproduction signal generator 450 may includea selector for selecting either the reproduction signal RFa3 or RFa4.

Hereinafter, comparison between the optical head device shown in FIGS.21B and 21C and the conventional optical head device regarding thejitter will be described. In the following comparison, the numericalaperture NA of the objective lens is 0.6, the wavelength λ of the lightis 0.660 μm, and the space Gp between two adjacent guiding grooves is1.48 μm. The center line 508 of the series of pits 502 in the firstaddress zone 504 and the center line 509 of the series of pits 502 inthe second address zone 505 are each off the center line of the track507 by 0.37 μm.

In the conventional optical head device, in which the information isreproduced based on a signal indicating the difference among the signalsobtained from each of the areas in the light detector defined by acentral division line of the aperture, the jitter obtained bycalculation is 6.4%.

In the optical head device shown in FIGS. 21B and 21C, the informationcan be reproduced in the following manner. The division line 302 dividesthe aperture equally into two. The aperture has a radius of 1. Thedistance between the division lines 301 and 302 is represented by d, andthe distance between the division lines 303 and 302 is also representedby d, where d=0.23. The aperture is divided into two detection areas bythe division line 301 and 303, and the information is reproduced basedon a signal indicating the difference between the signals obtained fromthe two detection areas, namely, signals RFa3 and RFa4. In such asystem, the jitter obtained by calculation is 1.4%. Thus, the opticalhead device shown in FIGS. 21B and 21C can improve the jitter by 5% ormore compared to the conventional optical head device.

As described above, in the optical head device shown in FIGS. 21B and21C, information stored in the form of a series of pits which arepositioned off the track can be reproduced with a sufficiently low levelof jitter. Accordingly, margin against disturbance and the like isincreased, and thus information such as addresses can be recorded to andreproduced from the optical disk in the form of pits with satisfactorystability.

With reference to FIGS. 22A and 22B, other another optical head devicehaving a different information reproduction signal generator from thegenerators shown in FIGS. 18 and 20 will be described.

FIG. 22A shows a part of the optical disk having a groove 501 and aseris of pits 502 on the information layer 108 (FIG. 1).

FIG. 22B schematically shows a light detector 351 and an informationreproduction signal generator 450.

The information reproduction signal generator 450 includes adifferential operation circuit 434. The differential operation circuit434 generates a reproduction signal RFa0 based on expression (25).

RFa0=t1−t4  (25)

The reproduction signal RFa0 is used for reproducing the informationstored in the first address zone 504 and the second address zone 505.

Hereinafter, comparison between the optical head device shown in FIG.22B and the conventional optical head device regarding the jitter willbe described. In the following comparison, the numerical aperture NA ofthe objective lens is 0.6, the wavelength λ of the light is 0.660 μm,and the space Gp between two adjacent guiding grooves is 1.48 μm, Thecenter line 508 of the series of pits 502 in the first address zone 504and the center line 509 of the series of pits 502 in the second addresszone 505 are each off the center line of the track 507 by 0.37 μm.

In the conventional optical head device, the jitter obtained bycalculation is 6.4% as described above.

In the optical head device shown in FIG. 22B, the jitter obtained bycalculation is 2.6%. Thus, the optical head device shown in FIG. 22B canimprove the jitter by nearly 4% compared to the conventional opticalhead device.

As described above, in the optical head device shown in FIG. 22B,information stored in the form of a series of pits which are positionedoff the track can be reproduced with a sufficiently low level of jitter.Accordingly, margin against disturbance and the like is increased, andthus information such as addresses can be recorded to and reproducedfrom the optical disk in the form of pits with satisfactory stability.

In the optical head device shown in FIG. 22B, it is not necessary toselect a signal to be output from the information reproduction signalgenerator 450 in accordance with whether the information to bereproduced is stored in the first address zone 504 or the second addresszone 505. Accordingly, the optical head device shown in FIG. 22B can berealized with a relatively simple circuit configuration.

With reference to FIGS. 23A and 23B, another optical head device havinga different information reproduction signal generator from thegenerators shown in FIGS. 18, 20 and 22 will be described.

FIG. 23A shows a part of the optical disk having a groove 501 and aseries of pits 502 on the information layer 108 (FIG. 1).

FIG. 23B schematically shows a light detector 352 and an informationreproduction signal generator 450.

As shown in FIG. 23B, the information reproduction signal generator 450includes a differential operation circuit 437, and adders 435 and 436.The differential operation circuit 437 generates a reproduction signalRFa00 based on expression (26).

RFa00=(t1+t3)−(t2+t4)  (26)

The reproduction signal RFa00 is used for reproducing the informationstored in the first address zone 504 and the second address zone 505.

Hereinafter, comparison between the optical head device shown in FIG.23B and the conventional optical head device regarding the jitter willbe described. The conditions are the same as described above withreference to FIG. 22B.

In the conventional optical head device, the jitter obtained bycalculation is 6.4% as described above.

In the optical head device shown in FIG. 23B, the jitter obtained bycalculation is 1.2%. Thus, the optical head device shown in FIG. 23B canimprove the jitter by nearly 5% compared to the conventional opticalhead device.

As described above, in the optical head device shown in FIG. 23B,information stored in the form of a series of pits which are positionedoff the track can be reproduced with a sufficiently low level of jitter.Accordingly, margin against disturbance and the like is increased, andthus information such as addresses can be recorded to and reproducedfrom the optical disk in the form of pits with satisfactory stability.

In the optical head device shown in FIG. 23B, it is not necessary toselect a signal to be output from the information reproduction signalgenerator 450 in accordance with whether the information to bereproduced is stored in the first address zone 504 or the second addresszone 505. Accordingly, the optical head device shown in FIG. 23B can berealized with a relatively simple circuit configuration.

EXAMPLE 7

In a seventh example according to the present invention, informationwhich is recorded at a position off the grooves of the optical disk usedas an information memory medium is reproduced.

An optical system of the optical head device used in the seventh examplehas substantially the same structure as the optical system shown in FIG.14 except for including a holographic element 120 in lieu of theholographic element 117 and including a light detector 155 in lieu ofthe light detector 154.

FIG. 24 shows a pattern for dividing the holographic element 120,detection areas of the light detector 155, and cross sections of thezero-the order light 108 a, the minus first-order diffraction light 108b and the plus first-order diffraction light 108 c on the light detector155.

The holographic element 120 is divided into a plurality of strip-shapedareas A, a, B, b, C, c, D, d, and X. These letters indicating thestrip-shaped areas also indicate the corresponding cross sections of thezero-the order light 108 a, the minus first-order diffraction light 108b and the plus first-order diffraction light 108 c. The minusfirst-order diffraction light 108 b obtained from the areas representedby capital letters A, B, C and D is collected farther from the minusfirst-order diffraction light 108 b obtained from the areas representedby lower case letters a, b, c and d with respect to the detection lens116. The light corresponding to the areas X is not diffracted and thusis entirely transmitted through the areas X as the zero-the order light108 a.

The minus first-order diffraction light 108 b is received by detectionareas 230 through 232, and the plus first-order diffraction light 108 cis received by detection areas 233 through 236. The zero-the order light108 a is received by a detection area 237. A focusing error signal isgenerated by a signal obtained in accordance with the amount of lightreceived by the detection areas 230 through 232.

The detection areas 233 through 236, respectively, generate signals t1through t4 in accordance with the amount of light received. Thedetection area 237 generates signal x0 in accordance with the amount oflight received. A tracking error signal THE for the groove of theoptical disk 105 (FIG. 14) is obtained based on expression (27).

THE=(t1+t4)−(t2+t3)  (27)

The ratio of the diffraction efficiency of the plus first-orderdiffraction light 108 c (obtained by the detection area other than thedetection areas X of the holographic element 120) with respect to thezero-the order light transmitted through the detection areas X isindicated by β. An information reproduction signal RF is obtained byexpression (28).

RF=t1+t2+t3+t4+β·x0  (28)

For reproducing information stored in the optical disk having thestructure shown in FIG. 19A, when the focal point of the light from theobjective lens is in the track 507 of the first address zone 504, theinformation stored in the optical disk in the form of the pits 502 isreproduced by obtaining signal RFa1 by the operation represented byexpression (29).

RFa1−(s1+s4)−(s2+s3+β·x0)  (29)

When the focal point of the light from the objective lens is in thetrack 507 of the second address zone 505, the information stored in theoptical disk in the form of the pits 502 is reproduced by obtainingsignal RFa2 by the operation represented by expression (30).

RFa2=(s1+s4+β·x0)−(s2+s3)  (30)

Through such a system, the optical head device shown in FIG. 24 providesthe same effect as the effect described in the sixth example. Asdescribed above, in the optical head device shown in the seventhexample, information stored in the form of a series of pits which arepositioned off the track can be reproduced with a sufficiently low levelof jitter. Accordingly, information such as addresses can be recorded toand reproduced from the optical disk in the form of pits withsatisfactory stability.

EXAMPLE 8

In an eighth example according to the present invention, characteristicsof a tracking error signal THE are improved using a holographic element.The focusing error signal is generated by the spot size method, and thetracking error signal is generated by the push-pull method.

FIG. 25 is a schematic view of an optical system of an optical headdevice according to the eighth example. The optical head device shown inFIG. 25 operates in the following manner.

Linearly polarized light emitted by a semiconductor laser 101 (used as alight source) is collimated by a collimator lens 103 as light 101 a, thecollimator lens 103 being included in a collection optical system. Thecollimated light 101 a enters a polarization anisotropic holographicelement 121 employed as a light division element. The holographicelement 121 is positioned so that the light 101 a from the semiconductorlaser 101 is not diffracted by the holographic element 121. The light 01a transmitted through the holographic element 121 is circularlypolarized by a ¼-wave plate 122. The light 101 a is then collected by anobjective lens 104 on an information layer 108 of an optical disk 105used as an information memory medium.

The light is reflected and/or diffracted by the information layer 108 ofthe optical disk 105 as light 108 a is transmitted back through theobjective lens 104 to be collimated. The collimated light 108 a istransmitted through the 1/4-wave plate 122 to be converted into linearlypolarized light which runs in a direction perpendicular to the directionof the light 101 a from the semiconductor laser 101. The linearlypolarized light is then diffracted by the polarization anisotropicholographic element 121 into minus first-order diffraction light 108 band plus first-order diffraction light 108 c. The minus first-orderdiffraction light 108 b and the plus first-order diffraction light 108 care converged by the collimator lens 103. Then, the minus first-orderdiffraction light 108 b is received by a light detector 156, and theplus first-order diffraction light 108 c is received by a light detector157.

A holding device 106 integrally holds the polarization anisotropicholographic element 121, the 1/4-wave plate 122 and the objective lens104. An actuator 107 moves the holding device 106 in accordance with thefluctuation or decentration of the optical disk 105.

FIG. 26 shows a pattern for dividing the polarization anistropicholographic element 121, detection areas of the light detectors 156 and157, and cross sections of the minus first-order diffraction light 108 band the plus first-order diffraction light 108 c respectively on thelight detectors 156 and 157.

The holographic element 121 is divided into a plurality of strip-shapedareas A, a, B, and b. These letters indicating the strip-shaped areasalso indicate the corresponding cross sections of the minus first-orderdiffraction light 108 b and the plus first-order diffraction light 108 con the light detectors 156 and 157. The minus first-order diffractionlight 108 b obtained from the areas represented by capital letters A andB is collected farther from the minus first-order diffraction light 108b obtained from the areas represented by lower case letters a and b withrespect to the collimator lens 103.

The optical system shown in FIG. 25 and the holographic element 121 aredesigned so that, when the focal point FO of the light from theobjective lens 104 is on the information layer 108 of the optical disk105, the cross section of the minus first-order diffraction light 108 bobtained from the areas represented by the capital letters A and B hasan equal size to that of the cross section of the minus first-orderdiffraction light 108 b obtained from the areas represented by the lowercase letters a and b.

Detection areas 238 through 243 respectively generate signals f1 throughf6 in accordance with the amount of light received. A focusing errorsignal FE is obtained b an operation represented by expression (31) or(32).

FE=(f 1+f 3+f 5)−(f 2+f 4+f 6)  (31)

FE=f 5−f 2  (32)

When the information layer 108 is distanced from the objective lens 104to be beyond the focal point FO of the light from the objective lens104, the cross sections A and B of the minus first-order diffractionlight 108 b are decreased, and the cross sections a and b of the minusfirst-order diffraction light 108 b are increased. Accordingly, theamplitudes of signals f1, f3 and f5 reduce and the amplitudes of signalsf2, f4 and f6 increase. The amplitude of the focusing error signal FEreduces.

When the information layer 108 approaches the objective lens 104 to bebetween the objective lens 104 and the focal point FO of the light fromthe objective lens 104, the cross sections A and B of the minusfirst-order diffraction light 108 b are increased, and the crosssections a and b of the minus first-order diffraction light 108 b aredecreased. Accordingly, the amplitudes of signals f1, f3 and f5 increaseand the amplitudes of signals f2, f4 and f6 reduce. The amplitude of thefocusing error signal FE increases.

In such a system, focusing control for maintaining the focal point FO onthe information layer 108 is realized.

Detection areas 244 and 245 of the light detector 157 respectivelygenerate signals t1 and t2 in accordance with the amount of lightreceived. The tracking error signal TE1 is obtained by expression (33)by the push-pull method.

TE 1=t 1−t 2  (33)

The tracking error signal TE1 substantially indicates the difference inlight amount between the lined areas and the blank areas of theholographic element 121 in FIG. 26.

An RF signal for reproducing the information is obtained as RFf byexpression (34), as RFt by expression (35), or as the sum of RFf andRFt.

RFf=f 1+f 2+f 3+f 4+f 5+f 6  (34)

RFt=t 1+t 2  (35)

Hereinafter, comparison between the optical head device shown in FIG. 26and the conventional optical head device regarding he degree ofasymmetry of the upper and lower amplitudes of the tracking error signalwill be described.

The optical disk includes a plurality of tracks having grooves or aseries of pits. The distance from the center of a track to the center ofan adjacent track is indicated by Tp. Where the numerical aperture ofthe objective lens 104 (FIG. 25) is NA and the wavelength of the lightis λ, expression (36) is fulfilled in the eighth example.

λ/(NA·Tp)≧1  (36)

A feature of the optical head device in the eighth example is that areasincluded in about 0.1 wide parts sandwiching the division line whichruns through the center of the aperture and is parallel to the groovesof the optical disk are exchanged with each other, and areas included inabout 0.1 wide parts at ends of the aperture are exchanged with eachother for the operation, as described in detail below.

In the case of a polarization anisotropic holographic element, thestrip-shaped areas A and a are alternately arranged in the part left tothe central division line 302, and the strip-shaped areas B and b arealternately arranged in the part right to the central division line 302.In the case of the holographic element shown in FIG. 26, onestrip-shaped area A and one strip-shaped area b sandwiching the centraldivisional line 302 are exchanged with each other. Along the ends of theaperture, the strip-shaped areas A and a are exchanged with thestrip-shaped areas B and b.

In the case where the strip-shaped areas are not exchanged as describedabove (i.e., in the conventional optical head device), the degree ofasymmetry of the upper and lower amplitudes of the tracking error signalis as follows. The degree of asymmetry is measured under the conditionswhere the numerical aperture NA of the objective lens is 0.5, thewavelength λ of light is 0.795 μm, the light intensity at the end of theobjective lens is 10% higher than the light intensity at the center ofthe objective lens, the thickness of the optical disk is 1.2 mm, and thedistance Tp between the centers of two adjacent tracks Tp is 1.6 μm.When the objective lens shifts by 500 μm, the degree of asymmetry is53%. When the radial tilt is 1.0 degrees, the degree of asymmetry is24%.

In the case of the optical head device shown in FIG. 26, the degree ofasymmetry is as follows where each of the strip-shaped areas has a widthof 0.1 times the radius of the objective lens and Tp is 1.6 μm. When theobjective lens shifts by 500 μm, the degree of asymmetry is 46%. Whenthe radial tilt is 1.0 degrees, the degree of asymmetry is 12%. As canbe understood, when the objective lens shifts by 500 μm, the degree ofasymmetry in the optical head device shown in FIG. 26 is 13% lower thanthat of the conventional optical head device. When the radial tilt is1.0 degrees, the degree of asymmetry in the optical head device shown inFIG. 26 is 50% lower than that of the conventional optical head device.Due to such a significant reduction in the degree of asymmetry, thetracking control is stabilized, and margin against disturbance and thelike is increased. Thus, information can be recorded and reproduced witha sufficiently low error ratio.

In an optical head device in which the aperture is simply divided intotwo, the difference between the focus position at which the amplitude ofthe information reproduction signal is maximum and the focus position atwhich the amplitude of the tracking error signal is maximum is 1.5 to1.0 μm. In the optical head device shown in FIG. 26, such a differenceis 1.0 to 0.5 μm. The reduction in such a difference also realizes alower error ratio for information recording and reproduction whilemaintaining the tracking control stable.

In the eighth example, the polarization anisotropic holographic elementis used as a light division element. Alternatively, a holographicwithout polarization anisotropy may be used. The same effect can beobtained.

In the eighth example, the polarization anisotropic holographic elementused as a light division element is driven integrally with the objectivelens. The light division element may be provided at any position betweenthe collection optical system and the light detector. As the objectivelens moves in accordance with the decentration of the track of theoptical disk or the like, the relative positions of the light divisionelement and the objective lens changes. By the pattern for dividing theholographic element described in the eighth example, the deteriorationof the tracking error signal due to this change can be restricted.

EXAMPLE 9

In a ninth example according to the present invention, the degree ofasymmetry of the tracking error signal is corrected. The optical systemof the optical head device used in the ninth example is identical withthat of the optical system shown in FIG. 25 except that a polarizationanistropic holographic element 123 is used in lieu of the holographicelement 121. The structure and operation of the optical system will beomitted.

FIG. 27 shows the holographic anistropic element 123 used in the ninthexample. The aperture is a circle having a radius of 1. Strip-shapedarea a each having a width of about 0.6 and strip-shaped areas B eachhaving a width of about 0.6 which sandwich a central division line ofthe aperture are exchanged with each other. The central division line isparallel to the track. A tracking error signal indicates the differencebetween the amount of light incident on the strip-shaped areas A and a(lined areas) and the amount of light incident on the strip-shaped areasB and b (white areas). In FIG. 27, two lined areas and two white areasare exchanged with each other.

The degree of asymmetry of the upper and lower amplitudes of thetracking error signal is as follows under the conditions where thenumerical aperture NA of the objective lens is 0.5, the wavelength λ oflight is 0.795 μm, the diameter of the objective lens is 4 mm, the lightintensity at the end of the objective lens is 10% higher than the lightintensity at the center of the objective lens, and the distance Tpbetween the centers of two adjacent tracks is 1.6 μm. In the case of theconventional optical head device in which the strip-shaped areas are notexchanged, when the objective lens shifts by 500 μm, the degree ofasymmetry is 53%. When the radial tilt is 1.0 degrees, the degree ofasymmetry is 24%.

According to one aspect of the optical head device shown in FIG. 27, forexample, the lined areas having a total width of 1.2 mm are exchangedwith the white areas having a total width of 1.2 mm with the centraldivision line at the center. When the objective lens shifts by 500 μm,the degree of asymmetry is 45%. When the radial tilt is 1.0 degrees, thedegree of asymmetry is 14%. As can be understood, when the objectivelens shifts by 500 μm, the degree of asymmetry in the optical headdevice shown in FIG. 27 is about 15% lower than that of the conventionaloptical head device. When the radial tilt is 1.0 degrees, he degree ofasymmetry in the optical head device shown in FIG. 27 is about 42% lowerthan that of the conventional optical head device.

The optical head device in the ninth example has a greater effectagainst the shift of the objective lens even than the optical headdevice shown in FIG. 26 in the eighth example. Accordingly, the trackingcontrol is further stabilized, and margin against disturbance and thelike is increased. Thus, information can be recorded and reproduced witha substantially low error ratio.

In the ninth example, the polarization anisotropic holographic elementis used as a light division element. Alternatively, a holographicwithout polarization anisotropy may be used. The same effect can beobtained.

In the first through ninth examples, an optical disk is used as aninformation memory medium. The same effect is achieved when an opticalcard or the like is used.

In the first through ninth examples, an infinite-type collection opticalsystem including a collimator lens and an objective lens is used. Thesame effect can be achieved with a limited type collection opticalsystem including an objective lens which also acts as a collimator lenswithout using a separate collimator lens.

EXAMPLE 10

In a tenth example according to the present invention, an inclinationdetection apparatus will be described.

FIG. 28 schematically shows an inclination detection apparatus in thetenth example. The inclination detection apparatus operates in thefollowing manner.

A linearly polarized scattering beam emitted by a semiconductor laser101 used as a light source is collimated by a collimator lens 103 andthen incident on a polarizing beam splitter 130 employed as a beambranching element. The beam is entirely transmitted through thepolarizing beam splitter 130 as a beam 70 and then transmitted through a1/4-wave plate 122 to be circularly polarized. The circularly polarizedlight is collected on an information memory medium 105 by an objectivelens used as a collection light system. The beam 70 is diffracted and/orreflected by the information memory medium 105 is transmitted backthrough the objective lens 104 and then through the 1/4-wave plate 122to be converted into a linearly polarized beam (also indicated byreference numeral 70) which runs in a direction perpendicular to thedirection of the light from the semiconductor laser 101. The beam 70 isthen entirely reflected by the polarizing beam splitter 130 and thenmade incident on a polarizing beam splitter 132. The beam 70 is dividedinto two beams 70A and 70B. The beam 70B is detected by a light detector159. The beam 70A is converged by a detection lens 133. The convergedbeam 70A is transmitted through a plane-parallel beam splitter 134 andthen received by a light detector 158. The beam 70A is provided with anon-point aberration for detecting a focusing error signal when passingthrough the plane-parallel beam splitter 134. The beam 70A received bythe light detector 158 and the beam 70B received by the light detector159 are respectively converted into electric signals in accordance withthe amounts thereof. The electric signals which are output from thelight detectors 158 and 159 are input to a signal processing section 700(FIG. 29).

FIG. 29 shows a configuration of the signal processing section 700.

The light detector 158 includes four detection areas 158A through 158D,and the light detector 159 includes two detection areas 159A and 159B.The signals which are output from the detection areas 158A and 158C arecurrent-voltage converted by a current-voltage converter 854, and thesignals which are output from the detection areas 158B and 158D arecurrent-voltage converted by a current-voltage converter 853. The signalwhich is output from the detection area 159A is current-voltageconverted by a current-voltage converter 852, and the signal which isoutput from the detection areas 159B is current-voltage converted by acurrent-voltage converter 851.

The difference between the signals output from the current-voltageconverters 853 and 854 is obtained by an operation section 874. Thesignal from the operation section 874 is output from a terminal 811 as afocusing error signal.

The difference between the signals output from the current-voltageconverters 851 and 852 is obtained by an operation section 871. Thesignal from the operation section 871 is output from a terminal 812 as atracking error signal.

The system for generating the focusing error signal, which is referredto as the non-point aberration method, and the system for generating thetracking error signal, which is referred to as the push-pull method, areboth known and will not described in detail.

The focusing error signal is sent to an actuator 107 for driving thefocusing control, and the tracking error signal is sent to an actuator107 for driving the tracking control.

The optical system and the information memory medium 105 are relativelypositioned so that the beam 70 from the semiconductor laser 101 isfocused on a desired position on the information memory medium 105.

The current-voltage converters 851 and 852 are added together by anadder 891. The signal from the adder 891 is sent to sample and holdsections 821 and 822. The sample and hold sections 821 and 822 performthe sample-and-hold operation respectively at the timing of timingsignals Sa1 and Sa2, which are generated by a trigger signal generator801. The difference between the signals from the sample and holdsections 821 and 822 is obtained by an operation section 872 and thenoutput from a terminal 813 as an inclination detection signal.

FIG. 30 shows the relationship between a pattern on the informationmemory medium 105 and the timing of the timing signal generated by thetrigger signal generator 801. In FIG. 30, letter x represents thedirection perpendicular to the track storing the information, and lettery represents the direction parallel to the track storing theinformation. Letter z represents the direction perpendicular to bothdirections x and y.

The information memory medium 105 includes first pattern areas havingmarks and spaces and second pattern areas having guide grooves Gn−1, Gnand Gn+1. The first pattern areas and the second pattern areas arearranged alternately in the direction y. In the second pattern areas,each represent a guide groove.

Letter Gp represents a distance between centers of two adjacent guidegrooves. Data can be stored in the guide grooves as well as between thegrooves in order to increase the amount of information which can bestored. Symbols Tn−2 . . . Tn+2 each represent a track for storinginformation. Where the distance between the centers of two adjacenttracks is tp, Gp and tp have the relationship of: Gp=2·tp. In thisexample, Gp=1.48 μm, the wavelength λ of the beam 70 from thesemiconductor laser 101 is 650 nm, and the numerical aperture NA of theobjective lens 104 (FIG. 28) is 0.6.

In the first pattern areas, marks 541 and 542 are formed with a distancefrom each other of ±Gp/4 in the direction x. The timing signals Sa1 andSa2 generated by he trigger signal generator 801 respectively correspondto the marks 541 and 542. The tracking error signal is generated using asignal obtained from the light detector 159 when the second patternareas are irradiated by the beam 70 collected by the objective lens 104.Where the signal output from the terminal 812 is a tracking errorsignal, when the angle made by the beams 70 collected by the objectivelens 104 and the information memory medium 105 changes, the signaloutput from the terminal 813 changes in correspondence with the change.

FIG. 31A is a schematic partial view of the track shown in FIG. 30. InFIG. 31A, (A) and (B) each represent a path of the beam collected by theobjective lens 104. When the beam runs the path (A), the signal which isoutput from the adder 891 (FIG. 29) has a waveform shown in FIG. 31B,and the signal which is output from the operation section 872 (FIG. 29)has a waveform shown in FIG. 31C. When the beam runs the path (B), thesignal which is output from the adder 891 has a waveform shown in FIG.31D, and the signal which is output from the operation section 872 has awaveform shown in FIG. 31E.

In the case of FIG. 31B, the values of the timing signals Sa1 and Sa2from the adder 891 are equal to each other. Accordingly, as shown inFIG. 31C, the output from the operation section 872 after the timingsignal Sa2 is output is zero. In the case of FIG. 31D, the values of thetiming signals Sa1 and Sa2 from the adder 891 are different from eachother. Accordingly, as shown in FIG. 31E, the output from the operationsection 872 after the timing signal Sa2 is not zero.

FIG. 32 is a graph illustrating the inclination detection signal outputfrom the terminal 813 with respect to the angle made by the beam 70collected by the objective lens 104 and the information memory medium105 in the case where Gp=1.48 μm and Gp=0.83 μm. In FIG. 32, theinclination is zero when the axis of the beam 70 collected by theobjective lens 104 is parallel to the direction z, i.e., perpendicularto the information memory medium 105.

In both of the cases where Gp=1.48 μm and Gp=0.83 μm, the inclinationdetection signal can be detected as long as the inclination of the angleis 1 degree or less. Such a level of detection sensitivity is 5 timeshigher than in the conventional inclination detection apparatus. Theinclination detection apparatus in the tenth example has such a highdetection sensitivity since the apparatus operates based on theprinciple that the phase of the beam is diffracted by the pattern andguide grooves of the information memory medium 105. The detectionsensitivity is higher when Gp=1.48 μm than when Gp=0.83 μm. Theapparatus utilizes the principle that detection sensitivity is relatedto the pattern of the information memory medium 105 and overlap of plusfirst-order diffraction light and minus first-order diffraction lightobtained from the light. The conditions under which the plus first-orderdiffraction light and the minus first-order diffraction light overlapare represented by NA>λ/Gp. The detection sensitivity of the inclinationof the angle is improved when the optical system has the relationship ofNA>λ/Gp.

The inclination detection apparatus in the tenth example realizesdetection of the inclination of the angle made by the beam collected bythe collection optical system and the information memory medium 105 witha higher precision than in conventional inclination detectionapparatuses. Moreover, since the inclination can be detected using alight detector for detecting a tracking error signal, a separate devicefor detecting the inclination is not necessary. Thus, a low-cost andcompact inclination detection apparatus can be provided.

In the tenth example, the signal which is output from the terminal 812is used as a tracking error signal and the signal which is output fromthe terminal 813 is used as an inclination detection signal.Alternatively, the signal which is output from the terminal 813 may beused as a tracking error signal and the signal which is output from theterminal 812 may be used as an inclination detection signal. Use of thesignal output from the terminal 813 is advantageous in, for example, anoptical head device without a driving section for correcting theinclination of the angle made by the beam 70 collected by the objectivelens 104 and the information memory medium 105. Even when such aninclination occurs, the positional deviation between the guide groovesand the track is substantially small, and thus the compatibility betweendifferent types of optical head devices and different types ofinformation memory mediums is improved.

In the case where the inclination detection signal is a control signalof a driving section 135 (FIG. 28) for driving the optical system andcontrols so that the beam 70 collected by the objective lens 104 and theinformation memory medium 105 made a prescribed angle, an optical headdevice for stably reading information even from a significantly curvedinformation memory medium. Through control of the intensity of the beamfor recording the information on the information memory medium 105 inaccordance with the inclination detection signal, satisfactory recordingof information can be performed on a significantly curved informationmemory medium.

In the tenth example, the light detector 158 for detecting a focusingerror signal and a tracking error signal and the light detector 159 forgenerating an inclination detection signal are separately provided.Alternatively, the optical system shown in FIG. 44 having one lightdetector for generating a focusing error signal and a tracking errorsignal as well as an inclination detection signal can be used.

EXAMPLE 11

FIG. 33 shows a configuration of a signal processing section 701 of aninclination detection apparatus in an eleventh example according to thepresent invention.

The signal processing section 701 is used in lieu of the signalprocessing section 700 shown in FIG. 29 in the inclination detectionapparatus.

The signal processing section 701 is different from the signalprocessing section 700 in that the former includes a sample and holdsection 823, a variable gain amplification section 831 and the operationsection 873 and has a different timing signal output from a triggersignal generator 802. The sample and hold section 823 performs thesample-and-hold operation at the timing of a timing signal Sa3 which isoutput from the trigger signal generator 802. As shown in FIG. 30, thetiming of the timing signal Sa3 corresponds to the space in the firstpattern area of the information memory medium 105. A signal which issampled and held by the sample and hold section 823 is, for example, asignal in proportion to the offset which is caused to the tracking errorsignal in an optical system for driving the objective lens for trackingcontrol as shown in FIG. 35 (infra) when the objective lens moves. Thesignal which is output from the sample and hold section 823 is, forexample, sent to the variable gain amplification section 831 andadjusted to have a desirable level. The signal output from the variablegain amplification section 831 is sent to the operation by the operationsection 873. The difference between the signal from the variable gainamplification section 831 and the signal from the operation section 871is obtained by the operation section 873. The signal output from theoperation section 873 is output from the terminal 812. By obtaining thedifference between the signal from the variable gain amplificationsection 831 and the signal from the operation section 871, the offsetcaused to the tracking error signal is eliminated even when theobjective lens moves by tracking control. Accordingly, stable trackingcontrol can be performed, and thus an inclination detection signal isaccurately detected.

EXAMPLE 12

FIG. 34 shows a configuration of an information memory medium in aninclination detection apparatus in a twelfth example according to thepresent invention. FIG. 35 is a schematic view of a signal processingsection 702 of the inclination detection apparatus.

The information memory medium shown in FIG. 34 is different from theinformation memory medium shown in FIG. 30 in that the former has aplurality of marks 541 and a plurality of marks 542.

Sample and hold sections 824 through 827 in the signal processingsection 702 sample and hold the signal from the adder 891 at the timingof timing signals Sa4 through Sa7. The timing signals Sa4 through Sa7respectively correspond to the marks 541 and 542 and mirror surfacesthereof. The timing signals are generated by a trigger signal generator803. The difference between signals output from the sample and holdsections 824 and 825 is obtained from an operation section 875, and thedifference between signals output from the sample and hold sections 826and 827 is obtained from an operation section 876. The differencebetween the signals from the operation sections 875 and 876 is obtainedby the operation section 872 and output from the terminal 813 as aninclination detection signal.

The inclination detection apparatus using the information memory mediumshown in FIG. 34 in the twelfth example provides an inclinationdetection signal at a higher level of detection sensitivity than theinclination detection apparatus using the information memory mediumshown in FIG. 30 in the tenth example.

EXAMPLE 13

FIG. 36 schematically shows an optical head device in the thirteenthexample. The optical head device operates in the following manner.

A semiconductor laser 101 used as a light source emits a light beam 70having a wavelength λ of 650 nm. The linearly polarized scattering beamemitted by the semiconductor laser 101 is collimated by a collimatorlens 103 and then made incident on a beam splitter 136 employed as abeam branching element. The beam splitter 136 is a half mirror, theoptical characteristics thereof do not rely on the polarizationdirection of the incident beam. Half of the intensity of the beam 70incident on the beam splitter 136 is transmitted through the beamsplitter 136 and then incident on a polarization filter 137.

FIG. 37 shows the polarization filter 137. The polarization filter 137includes two areas 137A and 137B. The area 137A allows transmission of100% of the light polarized in direction x but blocks 100% of the lightpolarized in direction y. The area 137B allows transmission of 100% ofthe light polarized in the direction x and also 100% of the lightpolarized in the direction y. The direction x is a radial direction ofthe information memory medium 105 (FIG. 36) and is perpendicular to thetangent to the track for storing the information. The direction y isparallel to the tangent to the track of the information memory medium105 and is perpendicular to the radial direction thereof. Direction x isperpendicular to both of the directions x and y and is parallel to theaxis of the beam 70.

In FIG. 37, reference numeral 70R denotes an image of the aperture ofthe objective lens 104 (FIG. 36). Reference numeral 70S indicates thesize of the area 137B. Since the size 70S is smaller than the image 70R,the effective numerical aperture NA of the objective lens 104 withrespect to the beam 70 polarized in the direction y (i.e., the effectivenumerical aperture NA of the beam 70 when being collected by theobjective lens 104) reduces. In the thirteenth example, the effectivenumerical aperture NA of the objective lens 104 with respect to thelight polarized in the direction x is 0.6, and the effective numericalaperture NA of the objective lens 104 with respect to the lightpolarized in the direction 7 is 0.4. A beam for which the effectivenumerical aperture of the objective lens 104 is 0.6 is referred to as afirst beam, and a beam for which the effective numerical aperture of theobjective lens 104 is 0.4 is referred to as a second beam. In order topolarize the beam in the directions x and y, a laser oscillating in boththe TE and TM modes can be used as the semiconductor laser 101. In thecase where a semiconductor laser oscillating only either the TE or TMmode is used, the beam can be polarized in the direction x or y bylocating the semiconductor laser 101 so that the polarization directionis slightly off the direction x or y. Alternatively, the beam emittedfrom the semiconductor laser 101 may be incident on the wave plate to becircularly or elliptically polarized. In this example, the semiconductorlaser 101 is position so that the polarization direction thereof isslightly off the direction x.

Referring again to FIG. 36, the beam 70 transmitted through thepolarization filter 137 is incident on the objective lens 104 included ithe collection optical system and collected on the information memorymedium 105. The beam 70 diffracted and/or reflected by the informationmemory medium 105 is transmitted back through the objective lens 104 andthen through the polarization filter 137. The beam 70 is then incidenton the beam splitter 136, which reflects the half of the intensity ofthe beam 70. The beam 70 reflected by the beam splitter 136 is incidenton a polarization beam splitter 130. The polarization beam splitter 130allows transmission of almost 100% of the light polarized in thedirection x (beam 70A), and reflects almost 100% of the light polarizedin the direction y (beam 70B). The beam 70B is detected by a lightdetector 159.

The beam 70A is converged by a detection lens 133. The converged beam70A is transmitted through a plane-parallel beam splitter 134 and thenreceived by a light detector 158. The beam 70A is provided with anon-point aberration for detecting a focusing error signal when passingthrough the plane-parallel beam splitter 134. The beam 70A received bythe light detector 158 and the beam 70B received by the light detector159 are respectively converted into electric signals in accordance withthe amounts thereof. The electric signals which are output from thelight detectors 158 and 159 are input to a signal processing section 704(FIG. 38).

FIG. 38 shows a configuration of the signal processing section 704.

The light detector 158 includes four detection areas 158A through 158D,and the light detector 159 includes two detection areas 159A and 159B.The signals which are output from the detection areas 158A and 158C arecurrent-voltage converted by a current-voltage converter 854, and thesignals which are output from the detection areas 158B and 158D arecurrent-voltage converted by a current-voltage converter 853. The signalwhich is output from the detection area 159A is current-voltageconverted by a current-voltage converter 852, and the signal which isoutput from the detection areas 159B is current-voltage converted by acurrent-voltage converter 851.

The difference between the signals output from the current-voltageconverters 853 and 854 is obtained by an operation section 872. Thesignal from the operation section 872 is output from a terminal 812 as afocusing error signal.

The difference between the signals output from the current-voltageconverters 851 and 852 is obtained by an operation section 871. Thesignal from the operation section 871 is output from a terminal 811 as atracking error signal.

The focusing error signal is sent to an actuator 107 for driving thefocusing control, and the tracking error signal is sent to an actuator107 for driving the tracking control.

The optical system and the information memory medium 105 are relativelypositioned so that the beam 70 from the semiconductor laser 101 isfocused on a desired position on the information memory medium 105.

The information recorded on the information memory medium 105 isobtained by adding the signals output from the current-voltageconverters 853 and 854.

The information memory medium 105 has guide grooves for realizingdetection of the tracking error signal. The distance Gp between centersof two adjacent guide grooves is 1.48 μm. In the case where theeffective numerical aperture NA of the objective lens 104 with respectto the first beam for reading information stored in the informationmemory medium 105 is 0.6 and the effective numerical aperture NA of theobjective lens 104 with respect to the second beam for detecting thetracking error signal is 0.4, substantially no phase shift occurs to thetracking error signal even when the angle made by the beam 70 collectedby the objective lens 104 and the information memory medium 105 isinclined from the normal angle. Accordingly, the off-track hardlyoccurs. The optical head device in the thirteenth example improves thecompatibility between different types of optical head devices anddifferent types of information memory mediums. A phase shift caused tothe tracking error signal when the angle made by the beam 70 collectedby the objective lens 104 and the information memory medium 105 isinclined from the normal angle is conspicuous when the distance Gp andthe numerical aperture NA have the relationship of Gp>λ/NA. Accordingly,the optical head device in the thirteenth example is configured to havethe relationship of Gp<λ/NA in order to obtain a satisfactory trackingerror signal.

In the optical head device in the thirteenth example, the two beams withrespect to which the effective numerical apertures of the objective lens104 are different are generated so as to have exactly the same axisunder any condition, by using the difference in polarization. Due tosuch a system, the adjustment required when assembling the optical headdevice in the thirteenth example is not more complicated than in thecase of the conventional optical head device despite radiation of thetwo beams toward the information memory medium 105.

Since the optical head device in this example has no restriction in themethod for detecting a focusing error signal, the focusing error signalmay be detected using the second beam. In such a case, the wave surfacefor realizing detection of the focusing error signal such as a non-pointaberration can be provided to the second beam. Since the effectivenumerical aperture NA of the objective lens with respect to the secondbeam is smaller than that with respect to the first beam, the aberrationon the wave surface is also smaller. In the case where the focusingerror signal is detected using the second beam, less noise is mixed tothe focusing error signal when the beam collected by the objective lenscrosses the track on the information memory medium than in the casewhere the focusing error signal is detected using the first beam.Accordingly, the focusing control can be more stable.

In the thirteenth example, the beam splitter 136 is a half mirror. Sincethe optical head device in this example is not influenced by thecharacteristics of the beam splitter 136 such as reflectance ortransmittance, a beam splitter having a transmittance of 70 to 90% and areflectance of 30 to 10% may be used. A beam splitter 136 formed of ahalf mirror is appropriate for an optical head device used only forreproduction since the signals from the light detectors 158 and 159 aremaximum. A beam splitter having a transmittance of 70 to 90% isappropriate for an optical head device used both for recording andreproduction since the amount of light running from the semiconductorlaser 101 to the information memory medium 105 increases.

In the thirteenth example, the information memory medium 105 hascontinuous guide grooves as a pattern for realizing detection of thetracking error signal. Alternatively, separate marks or guide groovesmay be formed on the information memory medium 105. In such a case, thesample and hold section can be provided on the input side of theoperation section 871 of the signal processing section 704.

EXAMPLE 14

FIG. 39 is a schematic view of an optical head device in a fourteenthexample according to the present invention.

In this example, focusing control and tracking control are performed bydriving the objective lens 104 by actuators 107 for focusing control andtracking control. The objective lens 104 and the polarization filter 137are integrally driven by the actuators 107.

The beam 70 diffracted and/or reflected by the information memory medium105 and then reflected by the beam splitter 136 is collected by thedetection lens 133. The beam 70 is converted by the detection lens 133and incident on a holographic element 138. The holographic element 138divides the beam 70 into a zero-the order beam 70C and two first-orderdetection beams 70D and 70E. The beams 70C, 70D and 70E are detected bya light detector 160.

FIG. 40 schematically shows the pattern of an off-axis fresnel zoneplate formed on the holographic element 138. When the beam 70 collectedby the objective lens 104 is focused on the information memory medium105, the first-order diffraction beam 70D is focused before the lightdetector 160, and the first-order diffraction beam 70E is focused beyondthe light detector 160. The diffraction efficiency of the holographicelement 138 relies on the polarization direction. The holographicelement 138 is designed so that, for the beam polarized in the directionx, the diffraction efficiency of the zero-the order light is 0% and thediffraction efficiency of each of the plus and minus first-orderdiffraction beams is 40%, and so that, for the beam polarized in thedirection y, the diffraction efficiency of the zero-the order light is100% and the diffraction efficiency of each of the plus and minusfirst-order diffraction beams is 0%. The pattern on the holographicelement 138 is formed by proton exchange of lithium niobate.

FIG. 41 shows the relationship between the detection areas of the lightdetector 160 and the beams 70C through 70E. The light detector 160 hasdetection areas 160A through 160H. The beam 70C is received by thedetection areas 160A and 160B, the beam 70D s received by the detectionareas 160C through 160E, and the beam 70E is received by the detectionareas 160F through 160H.

In the optical head device in this example, the signal processingsection shown in the thirteenth example with reference to FIG. 38 can beused as it is. The signal output from the detection area 160A is sent tothe current-voltage converter 852, the signal output from the detectionarea 160B is sent to the current-voltage converter 851, the signalsoutput from the detection areas 160D, 160F and 160H are sent to thecurrent-voltage converter 854, and the signals output from the detectionareas 160C, 160E and 160G are sent to the current-voltage converter 853.The method for detecting the focusing error signal used in this examplereferred to as the spot size detection method is known as well as thenon-point aberration.

In this example, the center of the objective lens 104 and the center ofthe polarization filter 137 always match each other by integrallydriving the objective lens 104 an the polarization filter 137 by theactuator. At this point, the second beam is collected on the informationmemory medium 105 with little aberration, and thus a tracking errorsignal with less phase shift and less offset can be obtained even whenthe angle made by the beam 70 collected by the objective lens 104 andthe information memory medium 105 is inclined.

Moreover, through use of the holographic element 138, the focusing errorsignal and the tracking error signal and the signal indicating theinformation stored in the information memory medium 105 can be detectedby one light detector, and thus a low cost optical head device can berealized.

According to the present invention, the following effects, for example,are obtained.

(1) Even when a shift of the objective lens or a radial tilt occur, thedifference between |TEmax−TEO| (difference between the absolute valuesof TEmax and TEO) and |TEmin−TEO| (difference between the absolutevalues of TEmin and TEO) is reduced. TEO is the value of the trackingerror signal obtained when the center of the track is irradiated by thelight, TEmax is the maximum value of the tracking error signal obtainedwhile the light crosses track in the radial direction, and TEmin is theminimum value thereof. Even when the off-track amount is corrected tozero, the degree of asymmetry of the upper and lower amplitudes of thetracking error signal can be restricted to a sufficiently low value.Thus, stable tracking control is performed.

(2) Even when the light collection point is off the track, theinformation stored in the track can be reproduced stably with asufficiently low error ratio.

(3) The fluctuation of the gain of the focusing error signal isrestricted to a sufficiently low level. Thus, stable focusing control isperformed.

As described above, the present invention provides an optical headdevice realizing correct information reproduction and stable informationrecording and erasing with a sufficiently low error ratio. Such anoptical head device has higher compatibility with different types ofoptical information processing apparatuses and different typesinformation memory mediums.

An inclination detection apparatus according to the present inventioncorrectly detects an inclination of the angle made by the beam collectedby the collection optical system and the information memory medium evenwhen the inclination is 1 degree or less.

An optical information processing apparatus according to the presentinvention realizes stable information recording to and reproduction froman information memory medium which is significantly curved.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. An optical information processing apparatus,comprising: an optical system for collecting a beam emitted by a lightsource onto an information memory medium; a light detector forgenerating a detection signal in accordance with a light amount of thebeam reflected by the information memory medium; a signal processingsection for processing the detection signal; and a correction sectionfor correcting wave front aberration on the beam collected by theoptical system, the wave front aberration being generated by an offsetof an angle made by the beam collected by the optical system and theinformation memory medium from a prescribed angle, wherein: theinformation memory medium has a second pattern area including groovetracks having grooves, the groove tracks being arranged with aprescribed gap therebetween, NA>λ/G_(p) where G_(p) is a distancebetween centers of two adjacent groove tracks, λ is a wavelength of thebeam emitted by the light source, and NA is a numerical aperture of apart of the optical system facing the information memory medium, theinformation memory medium further has a first pattern area includingtracks having a mark and space array, the tracks being arrangedadjacently to each other, the mark and space array having a mark and aspace, the mark and space array in the first pattern area includes afirst mark and space array and a second mark and space array, which arearranged to be away from a center of the groove tracks by a prescribeddistance in a direction substantially perpendicular to the longitudinaldirection of the tracks, the first pattern area and the second patternarea are alternately arranged on the information memory medium in alongitudinal direction of the tracks, and the detection signal includesa first detection signal and a second detection signal, the firstdetection signal is obtained by the light detector when the firstpattern area is irradiated by the beam collected by the optical system,the second detection signal is obtained by the light detector when thesecond pattern area is irradiated by the beam collected by the opticalsystem, and the signal processing section generates a first processingsignal based on the first detection signal, generates a secondprocessing signal based on the second detection signal, outputs one ofthe first processing signal and the second processing signal as atracking error signal and outputs the other of the first processingsignal and the second processing signal as an inclination detectionsignal.
 2. An optical information processing apparatus, comprising: anoptical system for collecting a beam emitted by a light source onto aninformation memory medium; a light detector for generating a detectionsignal in accordance with a light amount of the beam reflected by theinformation memory medium; a signal processing section for processingthe detection signal; and a correction section for correcting wave frontaberration on the beam collected by the optical system, the wave frontaberration being generated by an offset of an angle made by the beamcollected by the optical system and the information memory medium from aprescribed angle, wherein: the information memory medium has a secondpattern area including groove tracks having grooves, the groove tracksbeing arranged with a prescribed gap therebetween, NA>λ/G_(p) whereG_(p) is a distance between centers of two adjacent groove tracks, λ isa wavelength of the beam emitted by the light source, and NA is anumerical aperture of a part of the optical system facing theinformation memory medium, the information memory medium further has afirst pattern area including tracks having a mark and space array, thetracks being arranged adjacently to each other, the mark and space arrayhaving a mark and a space, the mark and space array in the first patternarea includes a first mark and space array and a second mark and spacearray, which are arranged to be away from a center of the groove tracksby a prescribed distance in a direction substantially perpendicular tothe longitudinal direction of the tracks, the first pattern area and thesecond pattern area are alternately arranged on the information memorymedium in a longitudinal direction of the tracks, and the detectionsignal includes a first detection signal and a second detection signal,the first detection signal is obtained by the light detector when thefirst pattern area is irradiated by the beam collected by the opticalsystem, the second detection signal is obtained by the light detectorwhen the second pattern area is irradiated by the beam collected by theoptical system, and the signal processing section generates a firstprocessing signal based on the first detection signal, generates asecond processing signal based on the second detection signal, outputsone of the first processing signal and the second processing signal as atracking error signal and outputs the other of the first processingsignal and the second processing signal as an inclination detectionsignal.
 3. A method for processing optical information using an opticalinformation processing apparatus including an optical system, a lightdetector, a signal processing section and a correction section, themethod comprising the steps of: (a) collecting a beam emitted by a lightsource onto an information memory medium, using the optical system; (b)generating a detection signal in accordance with a light amount of thebeam reflected by the information memory medium, using the lightdetector; (c) processing the detection signal, using the signalprocessing section; and (d) correcting wave front aberration on the beamcollected by the optical system, using the correction section, the wavefront aberration being generated by an offset of an angle made by thecollected beam and the information memory medium from a prescribedangle, wherein: the information memory medium has a second pattern areaincluding groove tracks having grooves, the groove tracks being arrangedwith a prescribed gap therebetween, and NA>λ/G_(p) where G_(p) is adistance between centers of two adjacent groove tracks, λ is awavelength of the beam emitted by the light source, and NA is anumerical aperture of a part of the optical system facing theinformation memory medium, the information memory medium further has afirst pattern area including tracks having a mark and space array, thetracks being arranged adjacently to each other, the mark and space arrayhaving a mark and a space, the mark and space array in the first patternarea includes a first mark and space array and a second mark and spacearray, which are arranged to be away from a center of the groove tracksby a prescribed distance in a direction substantially perpendicular tothe longitudinal direction of the tracks, the first pattern area and thesecond pattern area are alternately arranged on the information memorymedium in a longitudinal direction of the tracks, and the detectionsignal includes a first detection signal and a second detection signal,the first detection signal is obtained by the light detector when thefirst pattern area is irradiated by the beam collected by the opticalsystem, the second detection signal is obtained by the light detectorwhen the second pattern area is irradiated by the beam collected by theoptical system, and the signal processing section generates a firstprocessing signal based on the first detection signal, generates asecond processing signal based on the second detection signal, outputsone of the first processing signal and the second processing signal as atracking error signal and outputs the other of the first processingsignal and the second processing signal as an inclination detectionsignal.